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BUREAU OF MINES 
INFORMATION CIRCULAR/ 1989 



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In Situ Leach Mining 

Proceedings: Bureau of Mines Technology 
Transfer Seminars, Phoenix, AZ, April 4, 
and Salt Lake City, UT, April 6, 1989 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9216 



In Situ Leach Mining 

Proceedings: Bureau of Mines Technology 
Transfer Seminars, Phoenix, AZ, April 4, 
and Salt Lake City, UT, April 6, 1989 



By Staff, Bureau of Mines 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel J. Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 



1 






PREFACE 

In April 1989, the U.S. Bureau of Mines held technology transfer seminars on in situ 
leach mining at Phoenix, AZ, and Salt Lake City, UT. The papers presented at those 
seminars, as well as three reports of related research, are contained in this Information Cir- 
cular. The papers highlight various aspects of Bureau research on in situ leach mining, in- 
cluding a field experiment in copper leaching in Arizona and the use of computer programs 
to model physical and chemical parameters involved in in situ mining. 

The technology transfer seminar used as a forum for the transfer of this research is one 
of the many mechanisms used by the Bureau in its efforts to move research developments, 
technology, and information resulting from its programs into industrial practice and use. To 
learn more about the Bureau's technology transfer program and how it can be useful to you, 
please write or telephone: 

U.S. Bureau of Mines 

Office of Technology Transfer 

2401 E Street, NW. 

Washington, DC 20041 

202-634-1224 



CONTENTS 



Page 

Preface i 

Abstract 1 

Introduction 2 

In Situ Copper Mining Field Research Project, by Jon K. Ahlness and Daniel J. Millenacker 4 

Generic Design Manual: Cost Model for In Situ Copper Mining, by Joseph M. Pugliese 7 

Introduction to the Environmental Permitting Process for In Situ Copper Mining, by Daniel J. Millenacker 14 

Laboratory Core Leaching and Petrologic Studies To Evaluate Oxide Copper Ores for In Situ Mining, by Steven E. Paulson 

and Harland L. Kuhlman 18 

Methods for Determining the Geologic Structure of an Ore Body as It Relates to In Situ Mining, by Linda J. Dahl 37 

Computer Modeling Applications in the Characterization of In Situ Leach Geochemistry, by Dianne C. Marozas 49 

Modeling Infiltration to Underground Mine Workings During Block-Cave Leaching, by Robert D. Schmidt 58 

HydroLogic: An Intelligent Interface for the MINEFLO Hydrology Model, by Michael E. Salovich 67 

Predicting and Monitoring Leach Solution Flow With Geophysical Techniques, by Daryl R. Tweeton, Calvin L. 

Cumerlato, Jay C. Hanson, and Harland L. Kuhlman 73 

Enhanced Well-Drilling Performance With Chemical Drilling-Fluid Additives, by Patrick A. Tuzinski, John E. Pahlman, 

Pamela J. Watson, and William H. Engelmann 86 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


atm 


atmosphere, standard 


L/min 


liter per minute 


°C 


degree Celsius 


M 


molar 


c/lb 


cent per pound 


m 


meter 


cm 


centimeter 


mD 


millidarcy 


cm 2 


square centimeter 


MHz 


megahertz 


cm 3 


cubic centimeter 


min 


minute 


cP 


centipoise 


mL 


milliliter 


D 


darcy 


mL/min 


milliliter per minute 


ft 


foot 


mm 


millimeter 


ft 2 


square foot 


fim 


micrometer 


ft/ mi 


foot per mile 


/^mho/cm 


micromho per centimeter 


ft/min 


foot per minute 


mm/min 


millimeter per minute 


ft/s 


foot per second 


mol 


mole 


p 

& 


gram 


mol/L 


mole per liter 


gal/d 


gallon per day 


MPa 


megapascal 


(gal/d)/ft 2 


gallon per day per square foot 


ms 


millisecond 


gal/min 


gallon per minute 


mt 


metric ton 


g/cm 3 


gram per cubic centimeter 


mt/yr 


metric ton per year 


g/L 


gram per liter 


mV 


millivolt 


h 


hour 


pet 


percent 


Hz 


hertz 


ppm 


part per million 


in 


inch 


psi 


pound (force) per square inch 


Kbyte 


kilobyte (1,024 bytes) 


rpm 


revolution per minute 


kg 


kilogram 


s 


second 


km/s 


kilometer per second 


st/d 


short ton per day 


kW 


kilowatt 


st/yr 


short ton per year 


L 


liter 


wt pet 


weight percent 


lb 


pound 


yr 


year 


lb/st 


pound per short ton 


$/yr 


dollar per year 


L/h 


liter per hour 







The Bureau of Mines expressly declares that there are no warranties express or implied that apply to the software 
discussed in this report. By acceptance and use of said software, which is conveyed to the user without consideration by 
the Bureau of Mines, the user expressly waives any and all claims for damage and/or suits for or by reason of personal 
injury, or property damage, including special, consequential or other similar damages arising out of or in any way con- 
nected with the use of the software discussed herein. 



IN SITU LEACH MINING 

Proceedings: Bureau of Mines Technology Transfer Seminars, 
Phoenix, AZ, April 4, and Salt Lake City, UT, April 6, 1989 



By Staff, Bureau of Mines 



ABSTRACT 



As part of its research on advanced mining technology, the U.S. Bureau of Mines em- 
phasizes studies on in situ leach mining. This research enables the Bureau to develop 
technical information that can allow mining companies to adopt in situ leach mining 
methods. Research results should help to remove the technical and economic barriers cur- 
rently hindering the development of commercial in situ leach mining operations. 

This report contains papers summarizing results of Bureau research on various aspects 
of in situ leach mining. A featured topic is the cooperative field experiment of the Bureau 
and Santa Cruz Joint Venture in a copper deposit near Casa Grande, AZ. Discussions in- 
clude an overview of the entire project, cost modeling, environmental permitting, laboratory 
core-leaching experiments, petrographic studies, and an evaluation of the geologic structure 
in the region. In addition, papers describe hydrologic modeling and geophysical studies of 
solution flow during leaching. Preliminary geochemical modeling studies are also sum- 
marized. A final paper describes a nonionic chemical additive with potential for significantly 
reducing the high costs of drilling in in situ leach mining. 



INTRODUCTION 



During the 1980's, the U.S. minerals industry found it dif- 
ficult to compete with foreign mining companies in world 
markets. The U.S. copper companies were high-cost producers 
in a shrinking world market. Copper mining companies began 
restructuring in order to survive. The restructuring included 
considerable closures to match production with lower demand. 
Closing inefficient operations and adopting new processing 
technology enabled companies to boost productivity. This pro- 
ductivity, coupled with recent changes in the foreign exchange 
rates of the U.S. dollar, returned many U.S. copper producers 
to better fiscal health. 

The present favorable market conditions will not, 
however, last forever. Major foreign companies also cut costs 
with considerable restructuring. U.S. companies remain high- 
cost producers. For continued existence, the U.S. companies 
must be able to sell mineral products at the same price as do 
foreign companies and still make a profit. Low ore grades, 
high labor rates, strict regulatory constraints, higher concern 
over miner health and safety, and more costly environmental 
safeguards all force U.S. mining costs upward. During the 
next market contraction, U.S. companies will again be par- 
ticularly vulnerable. Most of the restructuring during the early 
1980's was a one-time cure; similar actions will not likely be 
available to these companies in another down market. The 
Bureau of Mines believes that the industry needs new 
technology to overcome the high cost of conventional mining 
operations. 

The Bureau recently expanded its efforts to provide 
technology that will help the minerals industry develop the 
necessary new mining and processing methods. Bold new 
methods of mining that improve worker productivity, 
minimize worker exposure to hazardous underground condi- 



tions, profoundly alter environmental impacts, and reduce 
waste have moved to the forefront of Bureau research. 

One of these new methods is called in situ leach mining. In 
situ leach mining involves circulating dilute chemical solutions 
through an ore deposit to dissolve the target metals. The solu- 
tions may be applied to the top of the ore body and allowed to 
seep down through it by gravity. Or the solutions may be 
forced through the ore between series of vertical injection and 
recovery wells (fig. 1). This method is sometimes referred to as 
"true" in situ leach mining. In yet another variation, leach 
solutions are trickled over rubbled ore in stopes or other 
underground openings. Regardless of the application method, 
the solutions containing the dissolved metals are then pumped 
out of the ground and to a plant for metal recovery. After 
removing the metals from solution, operators recycle the solu- 
tion back to the ore body. In situ leach mining thus combines 
mining and processing operations. 

In situ leach mining offers several economic, safety, and 
environmental advantages over conventional mining. The 
Bureau believes that the method requires less capital invest- 
ment. By eliminating ore extraction and crushing, in situ leach 
mining also saves labor and energy costs. Other benefits in- 
clude fewer health and safety risks as miners will not be ex- 
posed to the hazards of underground mining. The technique 
disrupts the environment less than does conventional mining; 
in situ leach mining removes little from the ground except the 
target metals. It also leaves a mine site that is easily reclaimed 
to its original condition by capping wells and dismantling the 
processing plant. 

The big question with the method is the metal recovery. 
Operators experienced low recovery from shallow rubbled 
deposits that were leached in the 1970's. Bureau experiments, 



Injection 
well 



Recovery 
well 



Processing plant 





Ore mineralization 



Leach solution 



Figure 1.— Cross section of an in situ leach mining system in which the ore is leached via a system of vertical wells. 



however, indicate that certain deposits should leach well. Such 
results underscore the necessity of conducting the tests and 
analyses described in this Information Circular before deciding 
whether to leach an ore body. The main environmental con- 
cern in in situ leach mining is the control and collection of 
leach solutions within the ore body. Developing control 
technology has been a major focus of the Bureau's research. 
The overall advantages provide two very important 
benefits to mine operators, particularly to copper mining com- 
panies: 

1. Production costs will be reduced, thereby making 
U.S. producers more competitive in the world market and 
reducing foreign import dependence. 

2. Copper can be recovered from small and/or low- 
grade deposits that are currently not economical to mine by 
conventional methods. 

The Bureau began studying methods of leaching ore left 
in the ground during the early 1970's. The early studies concen- 
trated on fracturing shallow copper ore bodies to improve the 
flow of leaching solutions. By the late 1970's, the Bureau 
focused its in situ leaching research on uranium deposits in 
shallow, permeable sandstones. These studies helped stimulate 
commercial operations. Early in the 1980's, the Bureau began 
evaluating techniques for in-place leaching of other metals, 
such as manganese and gold. Column leaching experiments, 
geologic characterization, and fluid flow modeling 
predominated. 

In 1986, the Bureau began a research program emphasiz- 
ing in situ leach mining of shallow to moderately deep copper 
oxide ores. Its goal is to provide industry with the technology 
to design an in situ copper leach mining operation for any 
specific deposit. A combined laboratory and field testing ef- 
fort was started to obtain site-specific data from two oxide 
copper deposits near Casa Grande, AZ. In support of the field 
work, the Bureau is conducting whole-core laboratory 
leaching experiments to determine the effects of microstruc- 
ture, ore and gangue-mineral relationships, flow rates, and 
other factors on copper recovery and acid consumption. 

The last Bureau of Mines technology transfer seminar on 
in situ leach mining technology was held in Denver, CO, in 
1981. It emphasized uranium. This Information Circular con- 
tains the results of the Bureau's copper in situ mining research 



over the past 5 yr and also includes the results of relevant cop- 
per block-cave leaching research. 

The first five papers discuss several important aspects of 
the Bureau's cooperative in situ copper mining research near 
Casa Grande, AZ. The first presents an overview of the 
research. The second describes the leach mine design manual 
with cost model developed by Science Applications Interna- 
tional Corp. under Bureau contract J0267001. The third ex- 
plains the potential environmental impact of in situ copper 
leach mining and the requirements of regulatory agencies in 
Arizona. The fourth describes the laboratory tests necessary to 
evaluate copper deposits for leaching. The fifth paper 
discusses the potential effects of the geologic structures on 
leaching results at the deposit chosen for the experiment near 
Casa Grande, AZ. 

The next paper discusses the potential applications of 
geochemical modeling for estimating chemical reactions that 
will occur during in situ leaching. The next two papers discuss 
the importance of hydrologic modeling studies in helping to 
understand the fluid flow during block-cave leaching. These 
papers describe the recently developed MINEFLO computer 
model and its usefulness in evaluating complex hydrologic 
problems. 

The Bureau is also investigating potential applications of 
geophysical systems to monitor leachate during in situ mining. 
Tomographic reconstruction of seismic cross-hole data to help 
detect leach solutions is described in the next paper. 

The final paper discusses the applicability of nonionic ad- 
ditives to improve drilling performance. Commercial in situ 
leach mining operators will spend much money on drilling 
wells. Successful application of this drilling additive will save 
considerable costs. 

Additional technical information can be obtained by con- 
tacting William C. Larson, supervisor of the Advanced Mini- 
ing Division at the Twin Cities Research Center, 5629 Min- 
nehaha Avenue S., Minneapolis, MN 55417, or by calling 
612-725-4690. Information concerning the Bureau's research 
programs on Advanced Mining Systems can be obtained by 
contacting Peter G. Chamberlain, program manager, 2401 E 
Street, NW., Washington, DC 20241, or by calling 
202-634-9885. 



IN SITU COPPER MINING FIELD RESEARCH PROJECT 



By Jon K. Ahlness 1 and Daniel J. Millenacker 2 



ABSTRACT 

To help meet the need of the domestic copper mining industry for new technology, the 
U.S. Bureau of Mines is conducting a field test of in situ copper mining at the Santa Cruz 
deposit near Casa Grande, AZ. A preliminary commercial mine design has been prepared for 
the site, based on a costing manual developed under Bureau contract. This paper describes 
the site and the well field design, as well as the steps involved in conducting the field test. 
These steps include obtaining geologic and hydrologic data on the ore zone, applying for en- 
vironmental permits, constructing the well field and a solvent extraction-electrowinning 
plant, startup, leaching and processing operations, and well field decommissioning. 
Geologic characterization of drill core from the deposit is currently under way, along with 
whole-core laboratory leaching experiments. Test site geology and hydrology data will be 
gathered for development of final project designs and environmental permit applications. 



INTRODUCTION 



In recent years, copper production in the United States 
has experienced a significant decline; however, it is once again 
increasing, owing to the recent increase in copper price. For 
example, domestic production of copper in the years 1977-81 
averaged 1.38 million mt/yr, while in the following 6 yr 
(1982-87) production averaged only 1.14 million mt/yr (a 
decrease of 18 pet) (7). 3 Production in 1987 was up to 1.27 
million mt, which is still down 8 pet from the 1977-81 average. 
Employment in the operating mines and mills fell from over 
30,000 people in 1981 to less than 10,000 in 1987. 

These decreases have occurred at a time when the United 
States is importing 27 pet of its copper requirements from 
Chile, Canada, Peru, Mexico, and other countries (2). Several 
factors are responsible for the drop in domestic production, 
including foreign competition, depletion of accessible, higher 
grade domestic reserves, and environmental considerations. 
Competition from developing countries is particularly intense 
in those cases where foreign reserves are higher grade, labor 
costs are low, and mines are nationalized or subsidized. 

Maintaining a viable copper industry in the United States 
is dependent on several factors. First, it is up to the copper 
producers to maintain the lower operating costs and improved 
efficiencies that have been developed in recent years. Second, 
the development of new lower cost, environmentally sound 
technologies (such as in situ mining) are required to maintain 
and, it is hoped, to enhance domestic copper production from 
the remaining small, low-grade, and/or deep deposits. The 



'Supervisory physical scientist. 

2 Hydrologist. 
Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN. 

J Italic numbers in parentheses refer to items in the list of references at the end 
of this paper. 



Bureau of Mines is conducting research to develop in situ min- 
ing technology to meet this need. 

In situ mining has been used on a commercial basis since 
the mid-1970's to produce uranium from porous sandstone 
deposits in Texas and Wyoming (5). In situ methods have also 
been used to recover copper from low-grade ore and waste 
rock in old open pit mines, block-caved zones, and backfilled 
stopes as an afterthought to conventional mining (4). Ore can 
also be placed on an impermeable surface in heaps and leached 
by sprinkling a leach solution (lixiviant) over the surface of the 
pile. Heap leaching is actively being used for extracting gold 
and silver from low-grade ore (J). Recovery of the pregnant 
leach solution in each of these operations occurs by fluid 
movement to a recovery well or underground sump, or to a 
surface collection pond. 

The Bureau's research objective is to stimulate domestic 
production of copper by the private sector, using in situ min- 
ing methods. To achieve this goal, the Bureau intends to pro- 
vide industry with the means to design the most economical in 
situ mining operation for any specific copper oxide ore 
deposit. A "Draft Generic In Situ Copper Mine Design 
Manual" has been prepared by Science Applications Interna- 
tional Corp. (SAIC), McLean, VA, under Bureau contract 
J0267001 (6). Subcontractors that assisted SAIC included Ray 
V. Huff and Associates, Golden, CO, Davy McKee Corp., San 
Ramon, CA, and Sergent, Hauskins, and Beckwith, Phoenix, 
AZ. This draft manual serves as a source document for 
developing a commercial-scale design for an in situ operation. 
The draft manual contains 

1 . A listing and description of the design elements and 
procedures for each component of an in situ copper mining 
system, and a method of costing individual components. 

2. A method of identifying the best of 42 possible in situ 
mining scenarios, which maximizes the discounted-cash-flow 



rate of return (DCFROR) for commercial operation for a 
specific site. 

3. A listing of the site-specific parameters that must be 
quantified to develop a commercial design and a description of 
laboratory and field tests to measure these parameters. 

4. A cash-flow model and computer program to con- 
duct a DCFROR economic analysis incorporating all capital 
and operating expenses associated with the well field, a solvent 
extraction-electrowinning plant (SX-EW), and environmental 
permitting. 



5. A description of the environmental procedures, 
specifications, designs, and costs for permitting, monitoring, 
and restoration. 

The generic approach provides flexibility in developing 
designs and costs for a wide range of deposit characteristics 
and operational parameters. To verify the draft manual and 
demonstrate the technical feasibility of in situ copper extrac- 
tion, a field test is being conducted in a copper oxide ore 
deposit in Arizona. 



IN SITU COPPER MINING FIELD RESEARCH PROJECT 



The Bureau is conducting the field research project in 
cooperation with the Santa Cruz Joint Venture (a mining part- 
nership formed between ASARCO Santa Cruz Inc., a sub- 
sidiary of ASARCO Incorporated, and Freeport Copper Co., 
a subsidiary of Freeport-McMoRan Gold Co.). This research 
is being conducted at the Santa Cruz site, 7 miles west of Casa 
Grande, AZ. A commercial mine design has been prepared for 
this site using the algorithms and model provided in the "Draft 
Generic In Situ Copper Mine Design Manual" (6). Actual field 
testing will utilize the unit-cell concept. A unit cell is a small 
well field constructed to commercial-scale well size and spacing 
specifications. Since the design and operational parameters are 
comparable to a commercial-scale operation, the field test will 
allow validation of the algorithms used in preparation of the 
commercial design and will also demonstrate whether an in situ 



facility can be operated as designed and within budget for a 
sustained period of time. Validation of the model will consist 
of comparison of actual versus calculated construction and 
operating costs, as well as verification of technical parameters 
(flow rate, fluid control, etc.). 

The Santa Cruz site (fig. 1) is undeveloped and contains a 
mineralized zone with estimated copper oxide reserves of 97 
million mt at an average grade of 0.7 pet acid-soluble copper. 
Chrysocolla and atacamite are the predominant copper oxides 
present in the granite and porphyry host rocks. Available field 
data indicate an in situ permeability range of 5 to 20 mD. The 
bottom of the mineralized zone is located at an average depth 
of 2,200 ft below the surface, with an average thickness of 345 
ft. Surface terrain is essentially flat lying with relief of 10 
ft/mi. Access to the site is provided by well-maintained State 




Figure 1.— Drill rig on the Santa Cruz site. 



and township roadways. Geologic characterization of 
available drill core is presently under way, along with 
laboratory whole-core leaching experiments. 

A well field of two connected five-spot patterns has been 
selected as the design basis for the field test. Corner wells in 
the pattern will serve production needs, and center wells will be 
used for injection. Producer-to-producer well spacing is 
designed to be 160 ft. The selected well spacing and calculated 
flow rate allow a sufficiently large area to be leached to deter- 
mine copper recovery, copper loading, and residence time ex- 
pected for a commercial operation in a representative geologic 
environment. Short-radius, propped hydraulic fractures may 
be necessary to increase the effective wellbore radius. 

The entire field test is scheduled to run for a 48-month 
period. Actual fluid injection-recovery operations will, 
however, run for only 18 months. Construction and decom- 
missioning activities account for the balance. The 18-month 
production period will allow sufficient time to assess opera- 
tional problems and to determine if copper loadings can be 
maintained over an extended period. 

Several steps are involved in conducting the field test. The 
first of these is to obtain hydrologic and geologic data from 
the ore zone and vicinity. A test program will be initiated to 
assess, first, flow characteristics within the selected ore block 
and, subsequently, flow control within a well pattern. Initially, 
two holes will be drilled to obtain core and to allow 
geophysical logging and open hole testing. Testing will provide 
data on porosity and permeability variations within the vertical 
ore interval. Upon completion of testing, two additional wells 
will be completed within 50 ft of the original wells. Tests will 
be conducted to determine orientation of the natural fractures 
(and effectiveness of wellbore stimulation) and to assess fluid 
movement and control within the well pattern. These data will 
be used for orienting the five-spot patterns. 

Environmental permitting activities will be initiated dur- 
ing the site investigation stage. Required ground water protec- 



tion permit applications will be prepared and submitted to the 
appropriate Federal and State regulatory agencies. Permitting 
is the mechanism used by the regulatory agencies to ensure that 
applicable water quality standards will be achieved in the areas 
adjacent to the facility during both operation and decommis- 
sioning. Requirements specific to protection of air resources 
will additionally need to be addressed at the local government 
level. Ground water protection requirements applicable to an 
in situ mining operation in Arizona have previously been 
discussed by Weeks (7). 

Upon completion of site characterization work, the well 
field will be constructed. The preliminary well field design 
assumes a 50-pct copper recovery over an approximate leach 
interval of 325 ft. The maximum flow rate expected for each of 
the injection wells is 40 gal/min. The pregnant solution grade 
is estimated to be 10 g/L. The final design (and parameter 
values) may be modified as field data become available from 
the site investigation. 

A solvent extraction-electrowinning (SX-EW) plant will 
be constructed upon completion of, or in conjunction with, 
well field installation. The plant will extract copper from the 
pregnant liquor, plate it as cathode copper, and recondition 
the lixiviant. The facility design will achieve an anticipated 
copper production rate of 1,095 st/yr cathode copper. 

Startup operations will initially occur over a 2-month 
period to establish flow rate equilibrium, determine steady- 
state solution concentrations, and identify and troubleshoot 
equipment problems. Upon completion of well field startup, 
the lixiviant will be injected and actual leaching operations will 
commence. Produced fluids will be pumped to the SX-EW 
plant for processing. 

Well field decommissioning will be initiated at the com- 
pletion of leaching. During this time, the surface plant will be 
dismantled and containment structures will be removed from 
the site. Wells will be abandoned and the leach interval 
restored as required by the regulatory agencies. 



SUMMARY 



The Bureau is conducting research to evaluate in situ min- 
ing as a method to maximize the probability of the domestic 
production of copper from small, deep, and/or low-grade cop- 
per oxide ore deposits. The Bureau intends to provide the U.S. 
copper mining industry with the technical and economic data 
necessary to design an in situ mining operation for any specific 
copper oxide ore deposit. A "Draft Generic In Situ Copper 
Mine Design Manual," prepared for the Bureau by Science Ap- 



plications International Corp., serves as a source document 
for developing a commercial design for an in situ operation. 
To verify the draft manual and demonstrate the technical 
feasibility of in situ extraction of copper, a field test is being 
conducted in a copper oxide ore deposit in Arizona. Field 
research is expected to continue for the next several years. 
Research results will be made available to the mining com- 
munity as significant milestones are attained. 



REFERENCES 



1. U.S. Bureau of Mines. Mineral Commodity Summaries 1977-87. 
Section on Copper. 

2. U.S. Department of the Interior. The Mineral Position of the 
United States— 1987. Annual Report of the Secretary of the Interior 
Under the Mining and Minerals Policy Act of 1970. BuMines Spec. 
Publ., 1987, 77 pp. 

3. Larson, W. C. Uranium In Situ Leach Mining in the United 
States. BuMines IC 8777, 1978, 68 pp. 

4. Ahlness, J. K., and M. G. Pojar. In Situ Copper Leaching in the 
United States: Case Histories of Operations. BuMines IC 8961, 1983, 
37 pp. 



5. Chamberlain, P. G., and M. G. Pojar. Gold and Silver Leaching 
Practices in the United States. BuMines IC 8969, 1984, 47 pp. 

6. Davidson, D. H., R. V. Huff, R. E. Weeks, and J. F. Edwards. 
Generic In Situ Copper Mine Design Manual (contract J0267001, 
Science Applications Int. Corp.). Volume II: Draft Generic In Situ 
Copper Mine Design Manual. 1988, 454 pp.; for inf., contact J. K. 
Ahlness, TPO, Twin Cities Res. Cent., BuMines, Minneapolis, MN. 

7. Weeks, R. E., and D. J. Millenacker. Environmental Permitting 
Considerations for True In Situ Copper Mining in the State of 
Arizona. Soc. Min. Eng. AIME preprint 88-196, 1988, 7 pp. 



GENERIC DESIGN MANUAL: COST MODEL FOR IN SITU COPPER MINING 



By Joseph M. Pugliese 1 



ABSTRACT 

The U.S. Bureau of Mines has provided a cost model for in situ copper mining that specifies 
(1) site-specific parameters, which must be quantified, (2) a method for individual site design based 
on these site parameters, and (3) a procedure for assessing the economic viability of the site-specific 
design. This paper describes the model, a menu-driven computer program that performs calcula- 
tions for developing commercial design specifications as well as capital and operating costs. The 
program also provides discounted-cash-flow rate of return (DCFROR) and sensitivity analyses for 
a true in situ copper mining operation at any specified site. This model can be used to compare the 
relative costs of different in situ mining scenarios. It provides a systematic method for assessing the 
commercial feasibility of applying true in situ copper mining at a selected site. 



INTRODUCTION 



The Bureau of Mines believes that the competitive position of 
domestically produced copper can be significantly improved with 
the application of in situ mining techniques. A long-term objective 
of the Bureau is to increase the probability of the domestic produc- 
tion of copper by the private sector, using in situ means. As part of 
the effort to meet this objective, the Bureau conducted research on 
cost modeling of in situ copper mining to provide the mining in- 
dustry with the means to design the most economically successful in 
situ copper operation for any specific deposit. The research was 
conducted by Science Applications International Corp. (SAIC), 
McLean, VA, under Bureau contract J0267001. The contractor had 
three subcontractors to aid in conducting the work: Ray V. Huff 
and Associates, Inc., Golden, CO; Davy McKee Corp., San 
Ramon, CA; and Sergent, Hauskins, and Beckwith, Phoenix, AZ. 



The contract effort started in October 1986, and the contractor's 
final report was completed in April 1988. ; 

The paper deals with the economic analysis found in Volume 
II of SAIC's final report and the commercial-scale operation exam- 
ple found in Volume IV of that report. Mention of these volumes 
and of SAIC's final report throughout the paper will be made with 
the understanding that the work is cited in footnote 2. 

The algorithms developed from this research are combined in- 
to the cost-modeling computer program for the design and 
economic analysis of an in situ copper mining operation. The com- 
puter program is on an IBM-compatible microcomputer; the 
minimum hardware requirements for running the program are a 
360-Kbyte memory and a math coprocessor. 



COST MODEL ASPECTS 



The cost model may be used to evaluate 42 in situ mining 
scenarios. Three basic characteristics are used to define each of the 
42 scenarios. 

The first characteristic addresses the way in which the ore body 
and adjacent rock will be penetrated, that is, the method of deposit 
access. Seven types of deposit access treatments are— 

1 . Drilling from the surface; 

2. Drilling from existing underground entries; 

3. Developing new underground entries, with drilling from 
those entries; 

4. Combinations of types 1 and 2; 

5. Combinations of types 1 and 3; 

6. Combinations of types 2 and 3; and 

7. Combinations of types 1, 2, and 3. 



The second characteristic involves the treatment of the 
rock matrix from within the borehole, or matrix modification. 
The three types of treatments considered by the cost model 
are — 

A. No modification; 

B. Modification using hydraulic fracturing; and 

C. Modification using explosives in the borehole. 



'Mining engineer, Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, 



MN. 



-Davidson, D. H., R. V. Huff, R. E. Weeks, and J 
per Mine Design Manual (contract J0267001, Science 
I: Executive Summary, Apr. 1988, 93 pp.; Volume 
Mine Design Manual, Apr. 1988, 454 pp.; Volume III 
Design of Commercial Scale Operation, Apr. 1988, 
Field Experiment and Design of Commercial Scale 
Volume V: Field Testing at the Santa Cruz Site, Apr. 
K. Ahlness, TPO, Twin Cities Res. Cent., BuMines, 



F. Edwards. Generic In Situ Cop- 
Applications Int. Corp.). Volume 
II: Draft Generic In Situ Copper 
Lakeshore Field Experiment and 
371 pp.; Volume IV: Sanla Cruz 
Operation, Apr. 1988, 385 pp.; 
1988, 1 14 pp.; for inf., contact J. 
Minneapolis, MN. 



The [bird characteristic involves the initial hydrologic set- 
ting of the ore body. Two types of treatments are — 

i. No hydrologic modification (already saturated); and 

ii. Hydrologic modification (unsaturated conditions 
originally). 

Type ii treatment indicates that the ore body should be 
saturated before leaching begins. 

Forty-two mining scenario permutations can be deter- 
mined from the types of treatments shown above. An example 
of a mining scenario permutation would be l.A.i., that is, 
drilling from the surface, no matrix modification, and an ore 
body that is initially saturated. The approach used to estimate 
which of the 42 possible mining scenarios may be best for a 
specific site is found in Volume II, chapter 9, of SAIC's final 
report. 



The cost model does the following: 

1. Develops detailed capital and operating costs for any 
proposed copper in situ mining project. 

2. Solves for pretax discounted-cash-flow rate of return 
(DCFROR) or selling price. 

3. Contains cost data (default values) from a broad ex- 
perience base. 

4. Allows for cost analyses where either minimum or 
maximum data are available. 

5. Accepts user-specified information. 

6. Conducts cash-flow analyses. 

7. Can be used for sensitivity analyses. 

Each of these features will be covered in the following sec- 
tions. 



COST MODEL INPUT PARAMETERS 



Input parameters are listed under the following 
categories: (1) business-related parameters, (2) site-specific ore 
body and well field characteristics, (3) copper leaching, (4) 
program control, (5) well system specifications and costs, (6) 
surface plant specifications and costs, and (7) environmental 
costs. The first three categories contain those input parameters 
that must be utilized to specify a certain mining scenario. The 
fourth category contains the input parameters that must be 
specified to control the printout of output data. The last three 
categories contain the input parameters that correspond to 
cost values and design specifications that translate a specific 
mining scenario into capital and operating costs. 

Category 1 consists of inflation indexes for plant and min- 
ing construction costs, copper selling price or rate of return, 
capital expenditure schedule, plant life, and copper produc- 
tion. 

Category 2 contains site-specific parameters that need to 
be considered in design of the wells and well pattern, such as 
depth to the bottom of the ore zone, thickness of the ore zone, 
leach interval, ore grade, permeability and porosity of the ore 
zone, type of mining access (surface or underground), status 
of underground mine access (existing or nonexisting), well 



field flow pattern, well spacing, type of well completion and 
stimulation, and ore body water saturation. 

Category 3 includes site-specific parameters that relate to 
pregnant liquor copper loading, acid consumption by gangue 
and copper minerals, and copper recovery efficiency. 

Category 4 contains the parameters that control whether 
to print out calculated variables, annual cash-flow summary, 
and/or all input parameters. 

Category 5 includes unit costs used to convert well system 
input parameters to output specifications and costs. Examples 
of these parameters are the cost of preparing the surface drill 
site; wellhead, packer, tubing, and casing costs; and hydraulic 
fracturing and explosive stimulation costs. 

Category 6 includes unit costs used to compute surface 
plant specifications and costs. These unit costs apply to such 
input parameters as pipes (lines), extractant, and diluent. Flow 
rate through the lines is covered under this category. 

Category 7 addresses environmental aspects of the in situ 
mining operation. Included here are monitoring well unit cost, 
cost of initial environmental permitting, annual cost of en- 
vironmental monitoring, and well field restoration cost. 



COST MODEL OUTPUT PARAMETERS 



Output parameters related to the well system, the surface 
plant, and the environment are grouped under the following 
categories: (1) design variables, (2) capital costs, and (3) 
operating costs. 

Category I includes leach area of unit cell; area of well 
field; injection pressure at top of ore zone; injection flow rate; 
total system flow rate; number of unit cells; well field life; in- 
jection acid concentration; acid and lime consumption; metal 
sulfates produced per year; number of injection and total wells 
(excluding monitor wells); well field solution transfer piping 
length and diameter; well-field-to-plant transfer piping 
diameter; total weight of copper in unit cell; fraction of copper 
recovered from the zone being leached; number of mine and 



plant operators; tubing, casing, and wellhole diameters; and 
injection and production pump power requirements. 

Category 2 includes well site preparation cost; well casing, 
cementing, drilling, completion, and stimulation costs; well 
logging cost; well injection and production equipment costs; 
cost per fan for fan wells; total surface facility capital cost; 
total cost of the solution transfer system; total well field 
capital cost; cost of initial environmental permitting; cost of 
monitoring wells; and well field restoration cost. 

Category 3 includes chemical and consumable costs, labor 
cost, cost of utilities, cost of environmental monitoring, and 
maintenance costs. 



CASH-FLOW ANALYSIS 



This cost model can be used to conduct a cash-flow 
analysis, using the DCFROR. A thorough discussion on 
DCFROR is found in Stermole, ! and a brief summary from 
that reference is presented in this section. The term "discount" 
is generally considered to be synonymous with "present worth" 
in economic evaluation work. The term "cash flow" is used to 
refer to the net inflow or outflow of money that occurs during 
a specified operating period such as a month or a year. The 
term "discounted cash flow" evolved from the idea that in- 
vestors often handle the time value of money using discounted 
(present value) calculations. That is, in evaluating the mining 
operation's economic potential, the investors used positive and 
negative cash flows at their present worth anticipated from an 
investment. 

According to the Stermole text, the investment cash flow 
in any one year represents the net difference between inflows 
of money from all sources minus investment outflows of 
money from all sources. This cash flow is calculated by 



Cfn 



R - CC - OC, 



where cf n 

R 

CC 

and OC 



net cash flow in each project year, $/yr, 
revenue, $/yr, 
capital costs, $/yr, 
operating costs, $/yr. 



The cost model developed by SAIC for the Bureau does not 
consider taxes, royalties, or predevelopment costs in the calcu- 
lations because of the generic nature of the cost model and the 
variation in these costs among States and companies. In addi- 
tion, the model does not include sunk costs, such as for ex- 
ploration and property acquisition; depletion allowances; 
depreciation; and salvage value. The cost model should be 
used to make relative production cost comparisons among dif- 
ferent commercial in situ copper mining scenarios for the pur- 
pose of selecting the one most economical. The cost model can 
form the basis for a mining company to develop its own ab- 
solute cost model. 

In using DCFROR, the term "net present value" (NPV) 
needs to be defined. ' The NPV is the cumulative present worth 
of positive and negative investment cash flow using a specified 
discount rate to handle the time value of money. As an equa- 
tion: 



NPV: 



E Cf n , 

n = (1 + i)" 



where NPV = net present value in year of calculation, $/yr, 
cf n = net cash flow in each project year, $/yr, 
n = year number (first year number = 0), 
N = total project life (plant life + construction 
period), yr, 
and i = discount rate, pet (expressed as a decimal in 
equation). 

The DCFROR is the value of "i" that makes NPV equal to 
zero. In the cost model, the DCFROR is used to calculate cop- 



Stermole, F. J., and J. M. Stermole. Economic Evaluation and Investment 
Decision Methods. Investment Evaluations Corp., Golden, CO, 1987, 479 pp. 
'Work cited in footnote 3. 



per selling price if DCFROR is specified. Conversely, copper 
selling price is used to calculate DCFROR if copper selling 
price is specified. 

The information required to define a commercial in situ 
copper mining operation for the Santa Cruz site near Casa 
Grande, AZ, for which design specifications and costs can be 
developed is found in Volume IV of the contractor's final 
report. In September 1986, the Bureau and the Santa Cruz 
Joint Venture (ASARCO Santa Cruz Inc. and Freeport Cop- 
per Co.) agreed to cooperate for the purpose of conducting 
data acquisition field tests and developing in situ mine designs 
for the Santa Cruz site. In May 1987, the Bureau contractor, 
SAIC, met with Asarco staff and obtained business decision 
and ore deposit information for use in defining a commercial- 
scale operation. In defining this operation, a scoping analysis 
was employed that did not use site-specific construction data 
pertaining to local Casa Grande conditions and did not con- 
sider local taxes and utility costs. 

Based on SAIC's analysis, the initial mining scenario (best 
from information available at the time) involved vertical wells 
drilled from the surface. Vertical wells seemed to be a reason- 
able place to start since no underground workings existed. For 
this Santa Cruz commercial-scale operation example, a five- 
spot pattern with a radial flow pattern to be used for leaching 
was developed (fig. 1). In this figure, the central injection well 
region is an expanded view showing the wellbore radius prior 
to small-radius hydraulic fracturing and the radius after this 
type of stimulation. A value of 8 ft is assumed as the effective 
wellbore radius, or radius after stimulation. The four producer 
wells are at the corners of the pattern and have a spacing (S) of 
160 ft. The depth to the bottom of the ore zone is 2,136 ft, and 
the ore zone thickness is 345 ft. The ore body is in a saturated 
state. The copper production rate (plant capacity) is estimated 
at 23,600 st/yr, and the plant life is 15 yr. The grade of ore is 
0.72 pet, and the permeability of the ore zone is estimated at 2 
mD. Underground copper recovery is assumed at 50 pet, and 
the copper loading is estimated to be 10 g/L. The other input 
and calculated parameter values for this Santa Cruz 
commercial-scale example can be found in Volume IV of 
SAIC's final report. 

Table 1 shows the annual cash flows for the commercial- 
scale copper in situ mining operation example at the Santa 
Cruz site and is taken from Volume IV of SAIC's final report. 
The DCFROR is 20 pet, which resulted in a copper selling 
price of 51 (50.94) c/lb. The project life is 18 yr, including the 
first 3 yr for construction, which results in a plant life of 15 yr. 
As well fields are depleted of copper, they are replaced with 
new well fields. In this example, well field construction is esti- 
mated to take 1 !/2 yr, and the well field life is 3 yr. As each well 
field is put into operation, a well priming time is required. For 
the years 4, 7, 10, 13, and 16, the lower production values 
shown reflect this priming effect. That is, each time a new well 
field is started up, no salable cathode copper is assumed to be 
available until 1 pore volume displacement has taken place. 
This lost production of cathode copper is subtracted from the 
normal annual production. The total operating and mainte- 
nance costs are incurred from the first year of production (year 
4) through the last year of project life (year 18). 



10 



KEY 

Wei I bore radius prior to stimulation 
T^ ] Effeotlve wellbore radius 
= S > Radial flow pattsrn 

^P Produoar well 
3 Produoer-well spacing 

Injeotlon-well 
region boundary, 




3 



Not to aoale 



Figure 1.— Plan view of five-spot pattern, commercial-scale operation example. 



11 



Table 1.— Annual cash flow, Santa Cruz commercial-scale operation example 

Approximate Production Costs, 10 3 $/yr Net cash Discounted 

Year production 1 revenue 2 Plant capital Well field Total operating flow cash flow 3 
(Y), (R), cost capital and maintenance (cf n ), (DCF), 
10 3 st/yr 10 3 $/yr (PCC) cost (WCC) cost (TOC) 10 3 $/yr 10 3 $/yr 

1 4,945 - 4,945 - 4,945 

2 5,374 2,187 -7,561 -6,301 

3 11,179 4,245 -15,424 -10,711 

4 18 18,286 11,908 6,378 3,691 

5 22 21,928 2,217 11,908 7,803 3,763 

6 22 21,928 4,304 11,908 5,716 2,297 

7 18 18,286 11,908 6,378 2,136 

8 22 21,928 2,217 11,908 7,803 2,178 

9 22 21,928 4,304 11,908 5,716 1,329 

10 18 18,286 11,908 6,378 1,236 

11 22 21,928 2,217 11,908 7,803 1,260 

12 22 21,928 4,304 11,908 5,716 769.3 

13 18 18,286 11,908 6,378 715.3 

14 22 21,928 2,217 11,908 7,803 729.3 

15 22 21,928 4,304 11,908 5,716 445.2 

16 18 18,286 11,908 6,378 414.0 

17 22 21,928 11,908 10,020 542.0 

18 22 21,928 11,908 10,020 451.6 

' Actual production: priming years, 17,949 st/yr; nonpriming years, 21,523 st/yr. On-stream plant operating time per year, 333 days (91.2 pet). 

2 Production revenue = actual production x 2,000 Ib/st x selling price. Selling price is 51 (50.94) c/lb, and DCFROR is 20 pet. 

3 Sum of these values = net present value (NPV) = 0. 

NOTE.— Underground capital cost (UCC) is $/yr for life of project. 

Source: Davidson, D. H., R. V. Huff, R. E. Weeks, and J. F. Edwards. Generic In Situ Copper Mine Design Manual (contract J0267001, Science Ap- 
plications Int. Corp.). Volume IV: Santa Cruz Field Experiment and Design of Commercial Scale Operation, Apr. 1988, 385 pp. 



SENSITIVITY ANALYSIS 



This cost model may be used to conduct sensitivity 
analyses. Sensitivity analyses can be used to evaluate the ef- 
fects of uncertainty on investment by determining how the in- 
vestment profitability varies as parameters are varied one at a 
time. 5 Further, sensitivity analyses may be used to identify 
those critical variables that, if changed, could affect substan- 
tially the profitability measure, such as DCFROR. When car- 
rying out a sensitivity analysis, the value for an individual 
variable is changed, and the effect of the change on the ex- 
pected rate of return (or other profitability measure) is 
calculated. Once the strategic variables that have major effects 
on the profitability measure are identified, they can be given 
special attention by the mine management. Several examples 
of parameters that may be allowed to vary in an in situ mining 
sensitivity analysis are copper loading, ore grade, well comple- 
tion and type of stimulation, and well spacing. These may be 
plotted against selling price while holding DCFROR constant 
or against DCFROR while holding the selling price constant. 

According to the Stermole book, the range approach to 
sensitivity analysis involves estimating the most optimistic and 
most pessimistic values (or the best and worst cases) for each 
parameter in addition to estimating most expected values. The 
best, worst, and most expected case sensitivity analysis results 
provide important information that bracket the range of proj- 
ect rate of return results that can reasonably be expected. 



'Work cited in footnote 3. 



Figures 2 and 3 represent examples of sensitivity plots for 
the Santa Cruz test site with data taken from Volume IV of 
SAIC's final report. In using sensitivity analysis, different 
values are assigned to one input parameter, plotted against the 
x-axis, while other input parameters are held constant. As dif- 
ferent values are given to the one input parameter, the corre- 
sponding changes in selling price are plotted against the y-axis. 
The selling price values are determined through the cash-flow 
analysis. For these sensitivity analysis examples, the DCFROR 
is 20 pet. The vertical line within each plot indicates the initial 
case value estimate for the parameter to be changed. 

In figures 2 and 3, the initial case parametric value pro- 
vides a selling price of 51 c/lb for the DCFROR of 20 pet. In 
general, in all of the examples an increase in the parametric 
value decreases the selling price. No estimates of ore grade 
above the initial case value of 0.72 pet were made (fig. 2C) 
because this value was thought to be at the high end for the 
Santa Cruz example. 

Figure 3, the plot of selling price versus permeability, is an 
illustration of combined effects in the cash-flow analysis. The 
permeability increases reduce the number of wells and thus the 
total cost of one well field. By maintaining a fixed well spac- 
ing, the well field will be depleted of copper in less time 
because of the higher injection flow rate. (More well fields will 
be needed over the project life, which will cause additional 
capital costs.) The higher flow rate (higher permeability) can 
result in higher negative discounted cash flow if the initial well 
field life is less than 3 yr, thus requiring a higher selling price to 
maintain the 20-pct DCFROR. 



12 












DO 

60 


i 


i i 

A 


DO 

60 




1 - 1" 


i 

B 








55 






55 












.a 
o 


50 






50 

«ft 












45 


i 


i i 




1 I 


i 






20 30 40 5 


)0 


125 150 175 


200 


LxJ 
O 

a: 

CL 


COPPER PRODUCTION RATE, 1,000 st/yr 




PRODUCTION WELL SPACING, ft 























o 



LU 
CO 




0.5 0.6 0.7 

ORE GRADE. * 



0.8 




5 10 15 

EFFLUENT COPPER LOADING, g/L 



20 



Figure 2.— Sensitivity analyses, Santa Cruz commercial-scale operation example. (Vertical line represents initial case.) 



70 
JQ 

s 

« 65 

•> 

Ld 
O 

a. 

o 

z 

Zj 55h 



60 



Ld 

to 



50 



\02) 


p r- i 

KEY 




(No.) Calculated 
well field life, yr 


W) 

i 


(2) (2) (2)- 





1 2 3 

PERMEABILITY, mD 



Figure 3.— Combined effects in cash-flow analysis, Santa Cruz commercial-scale operation 
example. (Vertical line represents initial case.) 



13 



SUMMARY 



A menu-driven computerized cost model was designed 
that may be used in developing a commercial in situ copper 
mining operation. Specifically, this cost model may be used to 
determine the economics of mining copper resources by the in 
situ mining method. This model permits the user to make cost 
inputs and inputs that are converted to mine design elements. 
Outputs that are generated include specific design parameters 
and capital and operating costs. Other outputs include a pretax 



selling price or DCFROR and a discounted-cash-flow table. 
The computer model may be used to make sensitivity analyses, 
whereby one input parameter's value is changed and the other 
input parameters' values are held constant. The effect of this 
change on selling price or DCFROR is then determined. The 
computer program is on an IBM-compatible microcomputer; 
the minimum hardware requirements for running the program 
are a 360-Kbyte memory and a math coprocessor. 



14 



INTRODUCTION TO THE ENVIRONMENTAL PERMITTING 
PROCESS FOR IN SITU COPPER MINING 



By Daniel J. Millenacker 1 



ABSTRACT 

This U.S. Bureau of Mines paper introduces the prospective mine operator to the en- 
vironmental permitting process for an in situ copper mining activity in the State of Arizona. 
It identifies the environmental aspects of in situ copper mining through a discussion of the 
regulatory authorization process. The Bureau intends to use this collected information in 
subsequent phases of its research to obtain permits to conduct a field test at the Santa Cruz, 
AZ, field site. 



INTRODUCTION 



In situ copper mining is a relatively new concept, which 
involves pressure injection of a dilute sulfuric acid leach solu- 
tion (lixiviant) into an ore zone through a geometrically ar- 
ranged well pattern. Dissolution of the mineralization located 
along fractures or disseminated within the ore occurs as the 
minerals are contacted by the injected lixiviant. Resulting 
copper-bearing solution is drawn to recovery wells where it is 
pumped to a surface processing plant. The solvent extraction- 
electrowinning (SX-EW) method is used to remove copper 
from solution. 

As part of its program to ensure a dependable domestic 
supply of minerals, the Bureau of Mines is evaluating the use 
of in situ methods for extracting copper from previously un- 
mined oxide ore deposits. Successful development of this min- 
ing technique and its acceptance by the mining industry will 
require an engineering design both technically feasible and en- 
vironmentally compatible. One task of the Bureau's research is 
to identify the environmental issues of in situ copper mining 
and the corresponding regulatory elements of environmental 
permitting and protection. 

Environmental issues specific to in situ copper mining are 
addressed through a permitting process administered by 
Federal, State, and local governmental regulatory agencies. 
Each agency maintains jurisdiction over a specific en- 
vironmental element of the mining operation. Permitting in- 
volves preparation of comprehensive application documents 
by the mining company and subsequent document submittal to 
the appropriate agency. Authorization to proceed with the 
mining activity may be either granted or denied by the 
regulatory agency depending upon anticipated facility com- 
pliance with environmental protection requirements. Once 
permits are approved and the facility is constructed, facility 
operation requires strict adherence to environmental protec- 
tion performance standards. It is incumbent upon the mine 
operator to consider all environmental requirements early in 



the mine planning process since permitting forms the first ma- 
jor step in establishing the feasibility, costs, and schedule for 
developing an environmentally sound mine design. 

Environmental resources afforded special protection 
through permitting include ground water, air, biological, and 
cultural resources. By the very nature of the in situ mining 
process, the greatest potential for environmental effect rests 
with the fluid injection-recovery system. Accordingly, the 
principal environmental permitting issue is protection of 
ground water resources. A permit application specific to 
ground water protection must include, at a minimum, infor- 
mation on site characteristics and hydrogeologic conditions, 
the extent of the area to be affected by the injection activity, 
anticipated effects to environmental resources, well and sur- 
face facility construction specifications and plans for opera- 
tion, site monitoring methods and procedures, contingency 
plans, and plans for facility closure. Federal and State re- 
quirements restrict injection activities that have the potential 
to degrade ground water resources. Requirements that apply 
to ground water protection in Arizona have previously been 
discussed by Weeks (/). 2 

Protection of air, cultural, and biological resources are 
subordinate to the issue of ground water protection. Air emis- 
sions expected to result from an in situ operation include 
fugitive dust from access road traffic, and volatile organics 
and sulfuric acid mist from operation of the SX-EW facility. 
Air resources protection requirements are intended to mini- 
mize the introduction of contaminants into the open air from a 
source or operation. Biological and cultural resources may be 
afforded special protection if ownership of "the property upon 
which the facility is sited is public. Given the potential for ef- 
fects on ground water and air quality from this type of mining 
activity, the focus of this paper is on permitting requirements 
as they apply to protection of these two environmental re- 
sources. 



'Hydrologist, Twin Cities Research Center, U.S. Bureau of Mines, Min- 
neapolis, MN. 



^Italic numbers in parentheses refer to items in the list of references at the end 
of this paper. 



15 



REGULATORY REQUIREMENTS IN ARIZONA 



Environmental requirements addressing protection of 
ground water and air resources in Arizona are currently ad- 
ministered by several governmental regulatory agencies. With 
respect to ground water protection, the U.S. Environmental 
Protection Agency (EPA), the Arizona Department of En- 
vironmental Quality (DEQ), and the Arizona Department of 
Water Resources (DWR) maintain jurisdiction. For injection 
activities occurring on Federal or Indian lands, requirements 
of the specific Federal land management agency (Forest Serv- 
ice, Bureau of Land Management, or Bureau of Indian Af- 
fairs) may also apply. Requirements of these agencies may be 
addressed in a facility plan of operation or other suitable docu- 
ment. Requirements addressing protection of air resources are 
administered through either DEQ or county air quality control 
boards. 



GROUND WATER PROTECTION 
Federal Water Quality Requirements 

EPA regulations relevant to fluid injection activities are 
authorized under the Underground Injection Control (UIC) 
program promulgated under part C of the Safe Drinking 
Water Act, Public Law 93-523. The technical requirements of 
this program are identified under 40 CFR 144 and 146 (2). Re- 
quirements specified under part 144 address permitting and 
program specifications. Part 146 addresses technical criteria 
and standards for operation. 

According to EPA, no fluid injection shall be authorized 
if it results in the movement of fluid containing a contaminant 
into an underground source of drinking water (USDW), if the 
presence of that contaminant may cause a violation of primary 
drinking water standards or may adversely affect public 
health. If an aquifer within a prospective leach interval meets 
the definition of a USDW, it must be determined to be an "ex- 
empted aquifer" before lixiviant injection may commence. An 
exempted aquifer is one that would otherwise qualify as a 
USDW but has been determined to have no real potential as a 
drinking water source. 

Five well classes have been established under the UIC pro- 
gram to regulate fluid injection activities. Two of the five well 
classes are applicable to in situ mining. A Class III well desig- 
nation applies to injection wells used for commercial in situ 
production from ore bodies that have not been conventionally 
mined. A Class V well is one used for solution mining of con- 
ventional mines, such as stope leaching, and for wells used in 
development of new technologies. The latter well class is 
generally assigned to those wells that inject nonhazardous 
fluids and are considered less of an environmental risk. An ex- 
ample of a Class V well is one used during a field-scale test of a 
mineral deposit to assess viability for commercial-scale in situ 
production. 

For in situ mining, the UIC program authorizes well con- 
struction by either permit or rule. Authorization by permit ap- 
plies to a Class III commercial well facility, while a Class V 
facility is authorized by rule. Application for a Class III well 
permit requires explicit and detailed information on the hydro- 
geologic resources present within the facility area, the well 
field design and operational plan, reporting and monitoring 
schedules, and plans for site closure. The principal emphasis 
of the Class III permit is design and operation of the facility to 
avoid migration of process fluids into, and degradation of, an 



overlying or adjacent USDW. The Class V authorization-by- 
rule process, on the other hand, requires an operator to pro- 
vide EPA with generalized inventory information on the facili- 
ty, along with any additional data that EPA may require. The 
EPA maintains the prerogative to require a UIC permit for 
any Class V well authorized by rule at any point in time if 
operation of that well does not meet minimum operating 
standards. 

In the application for a Class III permit, the physical 
limits of the study area must be established by defining the 
"area of review." This establishes the potential area of 
hydraulic influence of the well field. For a well array, the area 
of review is defined under 40 CFR 146 as the project area plus 
a circumscribing area the width of which is the lateral distance 
from the perimeter of the facility, in which the pressures in the 
injection zone may cause the migration of the injection and/or 
formation fluid into a USDW. EPA provides the option of ap- 
plying either this computational method for determining the 
area of influence or selecting a fixed radius of not less than 
one-fourth of a mile around the site. 

Closure of a Class III facility requires proper well field 
abandonment. The need to restore a leached interval is de- 
pendent upon the hydrologic conditions expected to return to 
the site upon completion of leaching activities. If hydraulic 
conditions are favorable for migration of residual process 
fluids from the site and the integrity of a receiving USDW can- 
not be assured, restoration will be necessary. Restoration is the 
rehabilitation of water quality within the leached interval to a 
baseline or predetermined water quality standard. A permittee 
is also required to maintain the financial resources to close, 
plug, and abandon the well field as required. Financial assur- 
ances (performance bonds, financial statements, etc.) must be 
provided to EPA as a component of the permit application. 

Arizona Water Quality Requirements 

The State of Arizona presently administers its own water 
quality protection program under authority of the Arizona En- 
vironmental Quality Act (EQA). The EQA was enacted by the 
State legislature in August 1986 to protect water resources 
from all activities that have the potential to degrade water 
quality. The DEQ was established as the principal agency re- 
sponsible for enforcing provisions of the EQA. Statutory 
language addressing protection of water resources from injec- 
tion activities is found under Arizona Revised Statutes (ARS) 
49-201 through 208, ARS 49-221 through 225, and ARS 49-241 
through 244 (5). The DEQ is required to adopt, by rule, an 
aquifer protection permit program to control discharges of 
any pollutant or combination of pollutants that are reaching 
or may with reasonable probability reach an aquifer. Efforts 
to establish this permit program are presently under way. With 
certain exceptions, an aquifer protection permit will be re- 
quired of any person who owns or operates a facility that dis- 
charges fluids. Given the recent date of statutory authoriza- 
tion, the rulemaking process will require 1 to 2 yr before final 
permit requirements are implemented. The DEQ is also re- 
quired to adopt, by rule, the UIC permit program described in 
the Safe Drinking Water Act. Any effort to obtain primacy 
from EPA for regulating underground injection activities will 
require the DEQ to adopt rules that closely parallel the EPA- 
UIC regulations. In developing its own UIC program, a State 
is not precluded from developing regulations more stringent 
than those established by EPA. Language specific to State 
UIC program requirements is identified under 40 CFR 145 (2). 



16 






Prior to enactment of the EQA, the Arizona Department 
of Health Services (ADHS) was responsible tor enforcing State 
\\ ater quality protection standards. (The ADHS was the prede- 
cessor to DEQ.) The ADHS regulated injection activities 
through a Notice of Disposal or ground water protection per- 
mit. Information requirements for a permit were more com- 
prehensive than those for a Notice of Disposal. Under ADHS, 
a permit application required information on site hydro- 
geology and anticipated hydrologic impacts, facility design 
and operation, monitoring, reporting, contingency planning, 
and decommissioning. The aquifer protection permit estab- 
lished under the new DEQ regulatory program will require 
consideration of many of the same elements originally in- 
cluded under the ground water protection permit. A new 
criterion, however, specifies that an operator apply the best 
available demonstrated control technology (BADCT) to en- 
sure aquifer protection. BADCT promotes the use of tech- 
nologies that result in discharge reduction or, if practical, 
complete elimination of the discharge. 

A key element of the new EQA specifies that all water- 
bearing units in Arizona meeting the definition of an aquifer 
are to be classified for drinking water protected use. The DEQ 
has defined an aquifer as a geologic unit that contains suffi- 
cient saturated permeable material to yield 5 gal/d of water to 
a well or spring (4). Water quality standards in effect for an 
aquifer are the primary maximum contaminant levels estab- 
lished by EPA. The DEQ maintains flexibility to establish 
aquifer water quality standards for additional pollutants for 
which no Federal or State standard exists. The classification of 
an aquifer, or portion thereof, can be changed by DEQ to a 
use other than drinking water if specific criteria are met. If a 
leaching operation is to occur in a geologic unit that meets the 
definition of an aquifer and water quality standards cannot be 
met at the designated facility point of compliance, the 
operator must petition DEQ for reclassification of the aquifer 
(or portion thereof) to a nondrinking water protected use. 

Arizona Water Conservation Requirements 

A second Arizona agency involved in the permitting pro- 
cess for an in situ copper mining operation is the DWR. The 



DWR is principally responsible for ground water conservation 
but also enforces stringent criteria for drilling, rehabilitation, 
completion, replacement, and abandonment of wells. Active 
management areas (AMA) have been established under DWR 
authority for regulating ground water use. Removal of ground 
water from a location within an AMA may occur only if a right 
to that water has been established. The DWR specifies pro- 
cedures to be followed in establishing a ground water 
withdrawal right. The DWR criteria specific to water use and 
well construction are defined under ARS 45-591 through 604 
(5) and R12-15-801 through 821 (6). 



AIR QUALITY PROTECTION 

Construction of an SX-EW plant will require an operator 
to obtain an air quality installation permit from either DEQ or 
an appropriate county air quality control board (7). (Arizona 
counties with approved State air quality control programs are 
given jurisdiction over facilities that produce a total emission 
quantity up to 75 st/d. Facilities producing emissions in excess 
of this rate are regulated by DEQ.) An installation permit is re- 
quired for each "permit unit" of basic equipment and its atten- 
dant air pollution control equipment. A permit unit contains 
all items of equipment that operate as a single functional unit. 
Additional permit units may be considered for storage of 
solids and liquids, incinerators, and fuel burning equipment. 
The installation permit application must provide a detailed 
description of all processes, process equipment, storage units, 
and fuels to be burned, and other information necessary to 
describe the proposed project and its air pollutant emission 
sources, together with estimated emission quantities. The per- 
mit applicant must describe the system or technique to be used 
to control emission of all pollutants and additionally provide a 
description of ambient air quality at the proposed site. An in- 
stallation permit remains in effect throughout facility con- 
struction and facility startup. Once the facility becomes com- 
mercially operational, an operating permit is granted by the 
regulatory agency if specified operational criteria are met. The 
operating permit establishes the emission standards to be met 
during facility operation. 



IN SITU COPPER MINING FIELD TEST 



The Bureau, in cooperation with the Santa Cruz Joint 
Venture (a mining partnership formed between subsidiaries of 
ASARCO Incorporated and Freeport-McMoRan Gold Co.), 
intends to conduct an in situ copper mining field test at the 
Santa Cruz site in the south-central part of Arizona, approx- 
imately 7 miles west of the town of Casa Grande. The project 
involves construction and operation of two interconnected 
five-spot well patterns and a surface plant capable of process- 
ing copper-bearing leach solution. The facility design was 
prepared using specifications calculated from the Generic In 
Situ Copper Mine Design Manual (8). The field test will utilize 
the unit-cell concept, which is essentially a small well field con- 
structed to commercial-scale well size and spacing specifica- 
tions. Corner wells in the pattern will serve production needs, 
and center wells will be used for lixiviant injection. The field 
test is scheduled to run for a 48-month period. Fluid injection- 
recovery operations will account for 18 months of this total, 
while construction, system testing, and decommissioning ac- 
tivities will account for the balance. Although the field test is a 
scaled-down version of a commercial-size operation, it is sub- 



ject to the same environmental analysis as would be required 
of a full-scale in situ copper mine. 

The Bureau, together with the Santa Cruz Joint Venture, 
has outlined three environmental tasks to be conducted during 
the 4-yr project life. The first of these is to establish a plan for 
permitting the field test. This includes developing permitting 
schedules and identifying the regulatory requirements and 
design criteria applicable to well field and surface plant con- 
struction and operation. The second task is to conduct a site 
hydrogeologic investigation to define in situ fluid flow condi- 
tions and to determine the area of hydraulic influence around 
the facility. Data collected through this investigation will be 
used both for permitting and for developing final engineering 
designs. The third and final task will be to construct the well 
field and surface plant and operate it in full compliance with 
permit terms and conditions. This task additionally involves 
closure of the facility in a manner consistent with permit re- 
quirements. 

At the present time, the first of the environmental tasks 
has been completed. Agency jurisdictions have been identified, 



17 



and permitting schedules have been estimated. Environmental 
permits that must be obtained include a Class V well 
authorization from EPA region 9, an aquifer protection per- 
mit from DEQ, a permit to construct wells from DWR, and an 
air quality installation permit from Pinal County, AZ. The 
Santa Cruz Joint Venture, as landowner of the property upon 
which the field test is to be sited, maintains a grandfathered 
ground water right and is free to withdraw water from the site 
without obtaining a separate ground water withdrawal permit 
from DWR. Design and performance criteria of each of these 
agencies have been incorporated into the preliminary facility 
design. 



A site hydrogeologic investigation, the second task, has 
been initiated but not yet completed. Collected data will be 
analyzed and used for development of the DEQ aquifer pro- 
tection permit, as well as for final facility design. The DEQ 
permit is by far the most difficult environmental authorization 
to be obtained for the field test. The estimated time required 
for permitting is 12 to 16 months. This period is all inclusive 
from initiation of the site investigation through issuance of the 
permit. Other agency permits will be sought in a manner con- 
sistent with the DEQ permit application schedule. The third 
task will be initiated during the second year of the field test 
and will carry through to field test completion. 



SUMMARY 



The Bureau is evaluating the feasibility of extracting cop- 
per from previously unmined oxide ore deposits using in situ 
methods. This research program gives equal weight to develop- 
ing a mining method both technically feasible and en- 
vironmentally compatible. 

The Bureau has identified the environmental re- 
quirements of in situ copper mining and the regulatory agen- 
cies that maintain jurisdiction over fluid injection activities in 
Arizona. Collected information is being used to develop a pro- 
gram for obtaining all necessary permits to conduct an in situ 
copper mining field test. The intent of the field test is not only 
to demonstrate the engineering feasibility of in situ mining but 



also to establish the environmental acceptability of this 
method through permitting, and to operate the facility in full 
compliance with applicable environmental protection per- 
formance standards. 

Although the environmental approach to this research is 
specific to in situ copper mining in Arizona, it can also apply 
to in situ extraction of other metals or to operations that might 
occur in other States. The experience gained from this en- 
vironmental program will provide the mining community with 
the information necessary to prepare a comparable and effec- 
tive program for permitting a future in situ mining operation. 



REFERENCES 



1. Weeks, R. E., and D. J. Millenacker. Environmental Permit- 
ting Considerations for True In Situ Copper Mining in the State of 
Arizona. Soc. Min. Eng. AIME preprint, 88-196, 1988, 7 pp. 

2. U.S. Code of Federal Regulations. Title 40 — Protection of En- 
vironment; Chapter I — Environmental Protection Agency; Sub- 
chapter D — Water Programs; Parts 144, 146 — Underground Injection 
Control Program; July 1, 1986. 

3. Arizona Legislature. Arizona Environmental Quality Act. 
Laws of 1986. Chapter 368, Arizona Revised Statutes, Title 49, 
Chapter 2, Articles 1-7; 1986. 

4. Arizona Department of Environmental Quality. Aquifer Boun- 
dary and Protected Use Classification. Arizona Administrative Rules 
and Regulations, Title 18, Chapter 11, Article 5; 1987. 

5. Arizona Legislature. Arizona Groundwater Code. Laws of 
1980. Fourth Special Session. Chapter 1, Arizona Revised Statutes, 
Title 45, Chapter 2, Articles 1-12; 1980. 



6. Arizona Department of Water Resources. Well Construction 
and Licensing of Well Drillers. Arizona Administrative Rules and 
Regulations, Title 12, Chapter 15, Article 8; 1984. 

7. Arizona Legislature. Arizona Environmental Quality Act. 
Laws of 1986. Chapter 368, Arizona Revised Statutes, Title 49, 
Chapter 3, Articles 2-3; 1986. 

8. Davidson, D. H., R. V. Huff, R. E. Weeks, and J. F. Edwards. 
Generic In Situ Copper Mine Design Manual (contract J0267001, 
Science Applications Int. Corp.). Volume I: Executive Summary, 
Apr. 1988, 93 pp.; Volume II: Draft Generic In Situ Copper Mine 
Design Manual, Apr. 1988, 454 pp.; Volume III: Lakeshore Field Ex- 
periment and Design of Commercial Scale Operation, Apr. 1988, 371 
pp.; Volume IV: Santa Cruz Field Experiment and Design of Com- 
mercial Scale Operation, Apr. 1988, 385 pp.; Volume V: Field Testing 
at the Santa Cruz Site, Apr. 1988, 114 pp.; for inf., contact J. K. 
Ahlness, TPO, Twin Cities Res. Cent., BuMines, Minneapolis, MN. 



18 



LABORATORY CORE-LEACHING AND PETROLOGIC STUDIES 
TO EVALUATE OXIDE COPPER ORES FOR IN SITU MINING 



By Steven E. Paulson 1 and Harland L. Kuhlman 2 



ABSTRACT 

This paper describes U.S. Bureau of Mines laboratory core-leaching and geologic characteriza- 
tion studies of oxide copper ores. These studies can provide detailed information about the nature 
of the various chemical and physical processes operating during leaching of oxide copper ores and 
determine the impact these processes will have on in situ mining. Three types of core-leaching 
systems are being utilized to conduct these experiments. The leaching experiments provide a very 
useful means of evaluating the effects of several variables on leaching, such as ore and gangue 
mineralogy and chemistry, textural relationships between ore and gangue minerals and fluid flow 
paths, leach solution concentration, flow rate, and pressure and temperature conditions during 
leaching. Results from two sets of experiments conducted on two different ore types from Pinal 
County, AZ, are discussed. These ores exhibited very different leaching characteristics because 
of their distinctly different mineralogy and texture. 



INTRODUCTION 



The Bureau of Mines has developed a laboratory research 
program with the main objectives of (1) gaining an understanding 
of both the physical and chemical factors that have an impact on 
the in situ mining of oxide copper ores and (2) developing techniques 
that can be used to evaluate ore deposits for their amenability to 
in situ mining. Three main research endeavors are utilized in this 
program: conducting laboratory core-leaching experiments, deter- 
mining the resulting fluid chemistry from these experiments, and 
conducting preleach and postleach petrologic studies. The infor- 
mation obtained from such studies on ore from a given deposit is 
combined with field data from that deposit to provide a better 
understanding of what can be expected if in situ mining is applied 
to the deposit. 

Up to this point, Bureau core-leaching studies have been con- 
ducted only on oxide copper ores from Arizona. The purposes of 
this paper are to explain in detail how the laboratory core-leaching 
experiments are conducted so that others may perform similar ex- 
periments on other oxide copper ores, to give examples of the types 
of data that are generated by such studies, and to illustrate the 
usefulness of these data for evaluating ore deposits for their suit- 
ability for in situ mining. 

There are two main reasons why the Bureau is conducting 
leaching experiments on core samples rather than crushed material. 
First, it is likely that the core-leaching experiments more closely 
simulate the nature of the solution-rock interface in situ. In many 
oxide copper deposits, the ore mineralization is distributed in the 
fractures and pore spaces of the rock, which are where the leach 
solutions will travel, both in situ and in core-leaching experiments. 
Thus, the fluid chemistry resulting from such experiments will more 



likely resemble that generated during in situ mining, giving a more 
realistic indication of what the fluid composition will be during in 
situ mining. In many cases, experiments performed with aggregate 
material would expose a much greater proportion of gangue 
mineralization to the leach solutions, perhaps resulting in a leachant 
chemistry significantly different from that generated during in situ 
mining. Therefore, it is likely that core-leaching experiments provide 
a better assessment of parameters such as copper concentration in 
solution, copper recovery from the rock, and acid consumption by 
gangue mineralization (7). 3 

Second, core-leaching experiments allow for an evaluation of 
permeability changes taking place during leaching. Influences on 
permeability include mineral dissolution and precipitation, clay 
mineral hydration, and physical sedimentation. Mineral dissolution 
may act to increase the size of the flow channels and thus increase 
permeability. However, the elevated sulfate (S0 4 -2 ) content in the 
leach solution may result in the combining of S0 4 -2 ions with 
cations mobilized by mineral dissolution or ion exchange with clay 
minerals, causing precipitation of sulfate minerals. This precipita- 
tion may decrease permeability. Also, permeability may be 
adversely affected by any carbon dioxide that may be generated 
by acid attack on any carbonate minerals in the rock. Although the 
permeability in a deposit will likely be much different from that 
measured during laboratory core-leaching experiments, correlation 
of permeability changes during the experiments with data on the 
fluid chemistry and sample mineralogy will yield information that 
may be useful for determining the potential impact of these 
mechanisms during actual in situ mining in the field. 

The three basic requirements for in situ mining are controlled 
by the inherent petrology and geochemistry of the ore body. First, 



■Geologist. 

Engineering technician. 
Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN. 



'Italic numbers in parentheses refer to items in the list of references at the end of 
this paper. 



19 



the leach solutions must be able to contact the ore minerals. This 
is controlled by the rock texture, the nature of the ore mineral 
distribution, and structural features of the ore body that influence 
the flow of leach solution through the deposit. Second, the target 
metals must be selectively solubilized by the leaching reagent. This 
is accomplished by the type of leach solution utilized and its con- 
centration. Finally, the metallic species must be mobilized to the 
recovery wells. This is influenced by the solution pH and the con- 
centrations of the various ions in solution, which are controlled by 
the reactivity of the minerals and the flow rate of the leach solution 
through the ore body. Precipitation may occur if ion concentration 
and/or solution pH reaches sufficiendy elevated levels. Metal ion 
mobilization can also be influenced by the interaction of clay 
minerals with the metal ions in solution, as metal ion sorption onto 
clay minerals can be significant in reducing metal ion concentra- 



tion in solution. Laboratory core-leaching and petrologic studies 
can help to determine what parameters have the greatest influence 
on these mechanisms, as well as the magnitudes of these influences. 
Among the variables being studied are the compositions of the ore 
and gangue minerals present in the system, the textural and struc- 
tual relationships among the minerals, the leach solution concen- 
tration, the flow rate of the leach solution through the rock (residence 
time), and the pressure and temperature conditions during the 
experiment. By incorporating the laboratory data with field data 
from a deposit, one can make predictions of what will actually occur 
during full-scale in situ mining of that deposit. Experiments will 
yield information such as copper loading in solution, fractional 
copper removal versus time, and lixiviant consumption per unit of 
copper produced. Such information is essential when assessing the 
feasibility of applying in situ mining to a deposit. 



EXPERIMENTAL APPARATUS 



The Bureau has a complete laboratory program to study the 
petrologic and chemical influences on in situ mining. This program 
includes the development of sample preparation techniques for both 
the petrologic studies and core-leaching experiments, as well as 
several different versions of core-leaching apparatus. Among the 
desirable features of a core-leaching system are flexibility to 
accommodate a variety of sample sizes and shapes; nonreactivity 
of the system with the leach solution; relatively low cost, thereby 
permitting a large number of samples to be tested; and ability for 
easy recovery of the postleach sample for petrologic studies (2). 

Three systems have been designed to conduct experiments under 
a variety of conditions. The main difference among the systems 
is the type of reaction cell used to house the core samples. 

Low-Pressure Core-Leaching System 
With Acrylic Reaction Cell 

The first type of reaction cell is constructed from acrylic pipe 
and can be used at pressures up to 50 psi. It requires a small amount 
of machining (figs. 1-2) and can be used again for subsequent 
experiments (2). Portions of the core samples selected for the 
experiments are submitted for chemical analyses, and thin sections 
are prepared for examination of the preleach geochemistry and 
petrology. The remainder of each core specimen is encased in epoxy 
prior to placement into the reaction cell. This is done because in 
many instances the samples become very friable because of reac- 
tion with the acid, leading to difficulty in preparing postleach thin 
sections. 

The steps in sample preparation are as follows. The sample 
is first painted with a thin coat of epoxy that cures at room 
temperature, has low shrinkage, and is not reactive with the sulfuric 
acid leach solutions used in these experiments (such as 3M 1838-L 
Epoxy Adhesive). 4 This thin coating fills all of the surface 
irregularities of the rock and ensures that the surface is completely 
sealed. The sample is then placed on end in a segment of polyvinyl 
chloride (PVC) pipe and the void space filled with epoxy. The inner 
surface of the pipe is coated with a silicone lubricant prior to the 
sample insertion to facilitate the removal of the sample after the 
epoxy has cured. The ends of the cylinder are trimmed to expose 
the surface of the rock and ground to provide a smooth surface 
perpendicular to the core axis. (This procedure can also be used 
for split core samples or other irregularly shaped pieces of ore 
material if whole core samples are not available.) 

After the sample is placed in the reaction cell, O-ring seal caps 
are placed against each end of the sample. The acid-resistant O-ring 



forms a seal between the wall of the reaction cell and the epoxy 
jacket encasing the sample, which forces the leach solution to travel 
through the core rather than along the outside edge (i.e. , short cir- 
cuiting). The seal caps contain a hole in the center through which 
the leach solution is pumped, and the seal cap surface facing the 
rock is slightly concave to allow access of the leach solution to the 



End cop 



O-ring 
seal cap 




PVC spacer 



Cell body 



Epoxy jacket 



Core sample 



Scale, in 



PVC spacer 



O-ring 



End cap 



"Reference to specific products does not imply endorsement by the Bureau of Mines. 



Figure 1.— Cross-sectional diagram of acrylic reaction cell for low- 
pressure (50 psi) core-leaching experiments. 



20 



entire rock faee. The necessary force to seal the O-ring at the sam- 
ple surface is provided by the end caps of the reaction cell. Screws 
are used to fasten the end caps to the main body of the reaction 
cell. The end caps are also fitted with O-rings to prevent leakage 
of leach solution from the reaction cell (fig. 1). If the core sample 
is shorter than the reaction cell, the excess space is taken up with 
a solid PVC cylinder of the same diameter, through which a hole 
has been drilled to permit transport of the leach solution. The solid 
PVC cylinder transfers the force from the end caps to seal the 
O-rings at the sample surface. Tubing and fittings made of Teflon 




Figure 2.— Two reaction cells for low-pressure core-leaching 
experiments. Pressure gauges assess permeability changes during the 
experiment. 



fluorocarbon polymer are used for the plumbing on the system, and 
the fluid samples are collected in closed polyethylene bottles (fig. 2). 

The leach solution is injected at the bottom side of the sample, 
and the injection pressure is monitored with a pressure gauge in 
order to assess permeability changes that may occur during the 
experiment (fig. 2). The gauges are fitted with plastic isolators to 
prevent reaction of the sulfuric acid solutions used in the experiments 
with the gauges. Reactions between the leach solutions and any com- 
ponents of the leaching system would lead to contamination of the 
fluid samples and, hence, severely complicate the study of the 
leachant chemistry and reaction mechanisms operating during 
leaching. A multichannel peristaltic pump is used to deliver the leach 
solution to the sample with this system (fig. 3). The pump delivers 
a constant flow of solution at low pressures (<30 psi), and the flow 
rate of each channel can be varied by changing the diameter of the 
flexible tubing that the metal rollers of the pump compress as they 
rotate. 

After completion of the experiment, the core sample is removed 
from the reaction cell, and the reaction cell can be reused. A portion 
of the sample is ground and analyzed for its postleach chemical 
composition. In most cases the sample is quite friable because of 
degradation during leaching. Therefore, the sample is dried and 
vacuum impregnated with epoxy so that the cutting and grinding 
needed for the preparation of the thin section can be easily 
accomplished. Usually, several thin sections are prepared from each 
sample to determine the extent of leaching within that sample. 

Medium-Pressure Core-Leaching System 
With PVC Reaction Cell 

The second type of reaction cell is constructed from PVC pipe 
and flanges (fig. 4). The main portion of this reaction cell can be 
used only once, but it is relatively inexpensive and requires very 
little machining. When this type of system is used, the sample is 




Figure 3.— Low-pressure experimental core-leaching system. This photograph displays six experiments being conducted simultaneously, using 
a multichannel peristaltic pump to pump the leach solution. 



21 



5E 



5 



ZE 



ir 



3 



E 



rr 



Blind flange 
Gasket 

Socket flange 

Flange socket 

PVC pipe 
Epoxy potting 
Test core 



PVC spacer 



3d 



5 



:e 



Figure 4. — Cross-sectional diagram of PVC reaction cell for core- 
leaching experiments. 



cast directly into the PVC pipe with epoxy. After the epoxy has 
cured, the ends of the whole assembly, which consists of core, 
epoxy, and pipe, are again trimmed and ground to provide two 
smooth surfaces. Each end is then fitted with a flange assembly, 
which consists of a flange, a flange socket, a gasket, and a blind 
flange (fig. 4). The flange itself is attached to the PVC pipe by 
means of a PVC epoxy adhesive, and the blind flange is attached 
with steel bolts. In order to reduce the volume of fluid stored in 
the reaction cell, the void space between the core sample and the 
blind flange is reduced by using a concave PVC spacer plate, 
through which a hole has been drilled (fig. 4). Again, as with the 
other type of reaction cell, a peristaltic pump and Teflon flurocarbon 
polymer tubing are used to deliver the leach solution to the core 
sample when working at pressures below 30 psi. 

The PVC reaction cell can be used at pressures up to 150 psi, 
but it is necessary to utilize a different pump and tubing when work- 
ing in this pressure range. Since the flow rate of the peristaltic pump 
is only constant at pressures up to 30 psi, a constant-flow, piston- 
type pump is used for higher pressure experiments. The pump 
selected for the current experiments is a constant-flow pump that 
can deliver fluid at a very low flow rate, if necessary (0.001 
mL/min) and can also be used at pressures up to 6,000 psi 
(fig. 5). This allows for both maximum sensitivity and flexibility. 
This pump was designed primarily for high-performance liquid 
chromatography applications. 

A pair of piston-type accumulators are used to isolate the pump 
from the leach solution. Each accumulator has a capacity of 500 
mL and is constructed of AISI Type 316 stainless steel (fig. 6). 
Water is pumped to drive the piston and force leach solution into 
the core sample. The two accumulators are connected in parallel 
so that as one delivers leach solution to the sample, the other can 
be refilled. A series of valves is used to isolate and switch the 
accumulators. Type 316 stainless steel tubing, fittings, and valves 
are used in place of Teflon fluorocarbon polymer at pressures greater 
than 100 psi. All stainless steel wetted parts are passivated with 
3M nitric acid for approximately 1 h prior to use in the system in 
order to further reduce the possibility of reaction with the experi- 
mental fluids. 

Upon completion of the experiments using the PVC reaction 
cell, one of two methods is used to remove the sample from the 
cell: The specimen is overcored using a core bit slightly larger than 




Figure 5.— Three experimental core-leaching systems utilizing PVC 
reaction cells and high-pressure constant-flow pumps. 



the sample diameter, or a lathe is used to trim away the PVC pipe 
from the sample. After the sample is recovered from the reaction 
cell, it is prepared in the same manner as described for the low- 
pressure system, with a sample submitted to the analytical chemistry 
laboratory before epoxy impregnation and thin section preparation. 

Elevated-Pressure Core-Leaching System 
With Stainless Steel Components 

A system has also been developed to conduct experiments at 
pressures above 150 psi. The maximum pressure at which this 
system can be used is 1,000 psi. There are two reasons for con- 
ducting experiments at these elevated pressures. First, the effects 
of pressure on the leaching characteristics of various ores need to 
be assessed, as it is likely that elevated pressures will be encountered 
during in situ mining of oxide copper ores because of the signifi- 
cant depths of some deposits. Therefore, elevated-pressure 
experiments would be conducted to determine to what extent, if 
any, pressure affects such parameters as primary mineral solubility, 
secondary mineral precipitation, and reaction kinetics. Second, many 
of the core samples tested in the laboratory exhibit very low 
permeabilities, thereby making it very difficult or impossible to 
establish an adequate flow of leach solution through these samples. 
Thus, a higher pressure is needed to force the leach solutions through 
the samples than can be attained with either of the two leaching 



22 



-*HI 



■End caps 



End plug 



■* Through bolts 

Barrel (316SS) 



Guide bushings 
Sealing ring 
Piston (316SS) 



Sealing ring 



Figure 6. — Cross-sectional diagram of stainless steel accumulator used 
with high-pressure pump to deliver leach solution to the core sample. 



systems previously described. This flexibility to conduct experiments 
over a relatively wide pressure range allows for a larger variety 
of samples to be tested and evaluated. 

Sample preparation for experiments using the high-pressure 
reaction cell is similar to that used with the PVC reaction cell. The 
core sample is placed upright in a piece of ANSI schedule 40 steel 
pipe, which is then filled with epoxy. After the epoxy has cured, 
the ends are trimmed and ground to produce smooth surfaces perpen- 
dicular to the axis of the sample. End caps for the reaction cell are 
composed of Type 316 stainless steel and are held in place with 
threaded steel rod passing through each cap (fig. 7). The end caps 
are machined to fit over the end of the steel pipe, and they are fitted 
with an acid-resistant O-ring, which forms the seal between the end 
cap and the epoxy jacket surrounding the core sample. The end 
caps are fitted with sintered titanium disks, which act as in-line filters 
(3) while also providing support to the sample ends, which 
frequently exhibit some spalling of material from the sample face. 
Passivated Type 316 stainless steel tubing, valves, and fittings are 
used throughout this leaching system. The 0. 16-cm tubing used has 
an inside diameter of only 0.06 cm in order to reduce the amount 
of fluid stored in the system. This small size also results in reduced 
surface area of the tubing exposed to the leach solution and reduced 
potential of back-diffusion of metals into the tubing and fluid 
reservoir. Back-diffusion may occur in experiments with very 
impermeable samples and, therefore, low flow rates. 

As with the PVC reaction cell, a constant-flow, piston-type 
pump and stainless steel isolators are used to deliver the leach solu- 







Epoxy with 
thickener 



Epoxy jacket 



Steel jacket 



Through bolts 



-rings 



Porous discs 



Figure 7.— Cross-sectional diagram of Type 316 stainless steel reaction 
cell for elevated-pressure core-leaching experiments. 



tion to the samples (fig. 8). The pump contains a high-pressure cutoff 
switch, which is triggered if the pressure reaches the preset upper 
pressure limit. At this point, the pump then becomes a constant- 
pressure pump rather than a constant-flow pump, until the pressure 
again drops below the preset upper pressure limit. The differential 
pressure between the injection and recovery sides of the sample 
is measured with a differential pressure transducer that is coupled 
to an electronic data recorder to provide a continuous readout of 
pressure throughout the experiment. The injection pressure of the 
pump is also monitored with the data recorder. 

If desired, a back pressure can be applied to the sample by 
means of compressed nitrogen gas. A pressure gauge is used to 
measure the actual back pressure applied to the sample. Fluid 
samples are collected in a Type 316 stainless steel collection vessel, 
which has been passivated with nitric acid (fig. 8). Experimental 
fluid samples are removed from the collection vessel on a regular 
basis and submitted to the analytical chemistry laboratory for 
analysis. As with the PVC reaction cell, the postleach rock sample 
can be recovered by overcoring or by turning down the pipe on 
a lathe. 



23 




Figure 8.— Elevated-pressure experimental core-leaching system (isolators not shown). 



LEACHANT RECIRCULATION EXPERIMENTS 



In addition to determining the fluid chemistry from experiments 
in which fresh leach solution is continuously injected throughout 
the duration of the experiment, it is also important to determine 
the effects on leaching chemistry of recirculating the leachants back 
through the rock after the copper has been removed, since this is 
what will actually occur during in situ mining. Previous experiments 
have shown that in addition to copper, appreciable amounts of 
aluminum, calcium, iron, potassium, and sodium can also be 
solubilized during leaching of oxide copper ores {1,4). Also, the 
amount of S0 4 " 2 in the system increases with time as additional 



sulfuric acid (H 2 S0 4 ) is added to the system to replace the acid con- 
sumed by gangue mineralization and to maintain the desired pH. 
Appreciable C 1 " can also accumulate in the leachant if a signifi- 
cant amount of a copper chloride mineral, such as atacamite, is 
present in the ore. Therefore, experiments were designed to study 
the effects of fluid recycling on parameters such as mineral solubil- 
ity, acid consumption, and net permeability. 

Leachant recirculation experiments are designed to examine 
the fluid chemistry after each pore volume of leach solution is 
pumped through the core. As a convention, pore volume is defined 



24 



here as the initial pore volume of the sample, since the actual pore 
volume may change during the experiment because of mineral 
dissolution and/or precipitation. Approximately 3 to 5 mL of sample 
is needed for each set of chemical analyses performed on the 
leachants. Therefore, it is desirable to recycle a total volume of 
solution that will not result in significant depletion of fluid because 
of sampling for the analytical determinations. Thus, in order to 
ensure that an adequate amount of fluid is available for the dura- 
tion of the experiment, a core sample with a pore volume >100 
mL is suggested. Since the porosity of many of the oxide copper 
ores tested in the laboratory ranged from 5 to 10 pet, a total rock 
volume between 1 ,000 and 2,000 cm 3 is recommended. Therefore, 
two types of experiments have been designed: one that utilizes a 
large-diameter core sample (e.g., 10 or 15 cm) and one that uses 
a longer segment of standard 5-cm-diameter core. 

A large-diameter core sample is more desirable than a smaller 
diameter sample because it has a greater surface area over which 
to inject the leach solution, thus enhancing the possibility of inter- 
secting mineralized fractures and fluid flow channels. Use of a large- 
diameter sample also decreases the length of core necessary to meet 
the minimum sample volume requirement, thereby allowing for a 
faster flow rate than in a smaller diameter core sample with the 
same permeability. This faster flow rate results in a reduction in 
the time necessary to complete an experiment. Since 10- or 15-cm 
core is not commonly available from exploratory drilling, larger 
blocks of ore obtained from such sources as open pit or underground 
mining operations need to be drilled to obtain these large-diameter 
core samples. However, it is possible to conduct such recycle ex- 
periments using smaller diameter core if larger samples are not 
available. 

Either the PVC or elevated-pressure reaction cell can be used 
for the recycle experiments. The same basic design is utilized, 
although the sizes of the components in the system need to be 
matched with the diameter of the core samples to be used in the 
experiments. After the diameter and approximate porosity of the 
core samples have been determined, the length of core needed to 
meet the minimum volume requirement can be calculated. In many 
cases it will be necessary to use several pieces of core in one reac- 
tion cell in order to meet the minimum sample size required to con- 
duct the recycle experiment. The ends of core samples selected for 
the experiment are trimmed on a rock saw and ground to provide 
flat and reasonably smooth surfaces. The samples are then stacked 
upright in the desired order, and a continuous composite rock core 



is created by using epoxy adhesive, thickened with amorphous 
fumed silica (3 wt pet), to spread around the joints between adja- 
cent core segments. The fumed silica, which is unreactive with the 
acidic leach solutions, gives the epoxy a pastelike consistency, which 
allows it to be spread across the joints between sample segments 
(fig. 7). Care must be taken to ensure that the thickened epoxy is 
not forced into the interface between the sample segments, as this 
will result in reduced solution flow through the composite sample. 
After this thickened epoxy has cured, the composite core sample 
is cast in epoxy and prepared as previously described. 

After an adequate amount of leachant has been collected and 
a 3- to 5-mL sample has been removed for the analytical chemistry 
determinations, one of two methods is used to remove the copper 
from the leachant prior to reinjection. The first method utilizes a 
small electroplating cell to plate the copper directly out of the 
leachant. The leachant is placed in a beaker and stirred as copper 
is plated onto a copper cathode. A platinum basket is used as the 
anode in this system. The second method of copper removal utilizes 
an organic solvent extractant. Examples of solvent extractants that 
worked well are Henkel LIX-622 and Acorga M-5397. A mixture 
consisting of two parts of kerosene and one part of extractant is 
added to an equal volume of leachant in a separatory funnel. The 
mixture is agitated for several minutes and then allowed to settle 
before the leachant is drawn off. This procedure is repeated until 
the leachant no longer exhibits any blue color (usually 2 to 5 times, 
depending on copper concentration). The leachant is then filtered 
through a 0.45-/im membrane filter to remove additional organic 
material that may be suspended. 

The process of removing copper from the leachant results in 
an exchange of H + for Cu +2 . However, the H + consumed by 
gangue mineralization is not returned to solution during the cop- 
per removal step. Therefore, it is necessary to restore the acid con- 
centration to its initial value. This can be achieved by adding either 
a relatively small amount of concentrated sulfuric acid or a larger 
amount of dilute sulfuric acid. Which option to use will be dictated 
somewhat by the total volume of solution being recycled. Adding 
concentrated sulfuric acid can cause a rapid increase in the S0 4 
concentration, which may be undesirable. However, the addition 
of dilute sulfuric acid may lead to undesirable dilution of the 
leachant. The advantages and disadvantages of both methods should 
be considered. Perhaps the most important point is to maintain con- 
sistency among experiments, as this will simplify data interpreta- 
tion and comparison. 



CHEMICAL AND PHYSICAL MEASUREMENTS 



Prior to leach solution injection, distilled, deionized water is 
pumped through the sample to completely saturate it and establish 
the flow rate conditions for the experiment. Fluid samples from 
the leaching experiments are collected, weighed, and analyzed on 
a regular basis, generally every 48 to 72 h, to thoroughly docu- 
ment the exact flow rate and chemical composition throughout the 
duration of the experiments. This sampling frequency was chosen 
to ensure that a large number of data points are obtained for each 
experiment. The acidity of each sample is measured using two dif- 
ferent methods. The first is a standard pH measurement and the 
second is an end-point titration technique to measure the free-acid 
content of the samples. Free acid is defined here as the H + present 
in the leachant, excluding any contribution due to cation hydrolysis 
after leaching (4). Because of the elevated levels of the various 
cations in solution, the potential exists for appreciable H + 
generation by hydrolysis reactions with these cations after the 
leachant is collected from the experiment. Such reactions may lead 
to erroneous assessments of acid consumption during leaching. 



Therefore, a potassium oxalate buffer solution is added to a small 
subsample of the leachant to complex the cations present. This solu- 
tion is then titrated with a base to determine the actual amount of 
H + remaining in the leachant collected, and therefore, how much 
of the initial acid was consumed by reaction with the rock. An 
automated titration system is utilized, which enables the rapid titra- 
tion of many samples. 

Approximately 2 to 3 g of leachant is diluted with distilled, 
deionized water, acidified, if necessary, to prevent any precipita- 
tion after dilution, and analyzed for the following components: 
dissolved silica, and aluminum, calcium, copper, iron, magnesium, 
potassium, and sodium. Atomic absorption (AA) spectroscopy and 
inductively coupled argon plasma (ICAP) spectroscopy are used 
to perform the analytical determinations for these components. An 
additional 0.25 to 0.50 g of solution is diluted and anlayzed for 
S0 4 ~ 2 and Cl~ using ion chromatography. Past experience has 
shown that the leachant chemistry from the experiments using oxide 
copper ores is composed chiefly of these 10 components. However, 



25 



the specific analytes to be measured should be determined after a 
close examination of the mineralogy of the rock samples to be used 
for the experiments. Also, when conducting recycle experiments, 
it may be necessary to expand the number of analytes, as elements 
present only in trace amounts in the rock may accumulate to 
appreciable concentrations after several recycle steps. 

Prior to the initiation of each experiment, a portion of the core 
sample is crushed, ground to pass a 100-mesh sieve, decomposed 
by a fusion or acid dissolution technique, and then analyzed for 
its elemental composition. ICAP and AA spectroscopy are used to 
measure aluminum, calcium, copper, iron, magnesium, manganese, 
potassium, silicon, sodium, and titanium. Combustion techniques 
are used to measure carbon and sulfur, while ion chromatography 
is used to measure chlorine in the rock. Trace elements such as 
antimony, arsenic, barium, cadmium, chromium, lead, mercury, 
selenium, silver, and zinc are measured using graphite furnace AA 
spectroscopy. 

Preleach and postleach petrologic and microprobe examina- 
tions of the ore samples provide a great deal of useful information 
about the texture and chemistry of the samples, which can be com- 
bined with the results of the leaching experiments to help explain 
the reactions occurring during leaching. Polished thin sections of 
the preleach and posdeach samples are prepared and examined using 
a polarizing microscope. Qualitative and quantitative chemical in- 
formation is obtained using an electron probe microanalyzer 
equipped with an X-ray analyzer. This system has the capability 
to analyze an area only a few micrometers in diameter, as well as 
X-ray mapping capabilities, which are used to display elemental 
distributions within a sample. 



In addition to the chemical analyses, porosity and permeability 
measurements are performed on the core samples used in the 
laboratory experiments. To determine the effective porosity of a 
sample, it is first dried under vacuum and weighed, followed by 
a vacuum saturation with distilled, deionized water and a second 
weighing. After the total volume of the sample is measured, the 
effective porosity is calculated using the volume of pore water after 
saturation. The permeability of each sample is determined after the 
injection of distilled, deionized water is initiated at the beginning 
of the experiment. The following equation, derived from Darcy's 
law, is used to calculate the permeability (5): 

K = nVL , 
APt 

where K = permeability, D, 

n = viscosity of fluid at measured temperature, cP, 

V = volume of fluid, cm 3 , 

L = length of sample, cm, 

A = cross-sectional area of sample, cm 2 , 

P = pressure, atm, 
and t = time, s. 

The field permeability of a particular ore deposit will most 
surely be quite different from that measured in a core sample in 
the laboratory, owing to the appreciable impact of large fractures 
and faults occurring in the deposit. However, the main purpose of 
measuring and monitoring permeability during the experiments is 
to assess the relative changes in permeability that occur as the 
experiment progresses, rather than to focus on the absolute 
permeability of the core samples. 



EXPERIMENTAL RESULTS AND DISCUSSION 



Fluid chemistry data generated in the core-leaching experiments 
are evaluated to provide input for making decisions in the field on 
such parameters as the number of wells operating at any given time, 
well spacing, and residence time of the fluid in the ore body. These 
three parameters have an effect on the copper loading in solution 
and the recovery rate of copper from the deposit. The minimum 
copper concentration in solution necessary to ensure the efficient 
operation of the surface recovery facility is maintained by controlling 
these three parameters. Among the mechanisms that influence the 
copper recovery rate are (1) chemical kinetics of mineral-acid reac- 
tivity, (2) acid and copper diffusion through reaction product layers, 
(3) acid and copper diffusion between the main flow channels and 
the other pores and microfractures in the rock, (4) chemical 
equilibria affecting the mobilization of copper to the recovery wells 
after dissolution, particularly pH controls, and (5) adsorption of 
mobilized copper onto mineral surfaces in the rock [Davidson (6)]. 
Core-leaching experiments provide a measure of the net effect of 
these mechanisms, which can then be used as input for the design 
of a true in situ mining well field. 

Some examples of data from both the core-leaching experiments 
and the petrologic studies are presented here to illustrate the type 
of data such studies yield and how this information is evaluated 
with respect to in situ mining. Each ore deposit displays its own 
unique characteristics; however, if the fundamental reaction 
mechanisms operating during leaching can be identified and quan- 
tified in the laboratory, then the evaluation of a particular deposit 
for in situ mining will be simplified. Thorough petrologic and elec- 
tron microprobe studies of the rock samples both before and after 
the experiments provide information that is very useful in explain- 
ing the resulting leachant chemistry and identifying the various reac- 
tions operating during leaching. 



To illustrate the significance of mineralogy and texture on the 
leaching characteristics of an ore, results of experiments using two 
oxide copper ores with distinct differences in their texture and 
mineralogy are presented, which show the very different reactivities 
that these ores exhibit. Both ores were from deposits in Pinal 
County, AZ. 

Chrysocolla Hosted in Granodiorite Porphyry 

The first type of ore studied was a granodiorite porphyry that 
contained chrysocolla as the principal copper mineral. The porphyry 
is significantly altered, resulting in a permeable, porous rock. 
Thorough petrologic studies of many thin sections of preleach 
samples determined that the average mineralogy of this ore was 
33 pet quartz, 30 pet plagioclase feldspar (one-half altered to copper- 
bearing clay), 16 pet potassium-feldspar, 12 pet biotite (one-half 
altered to chlorite and clay), 6 pet chrysocolla in fractures, and 3 
pet limonite. Chemical analyses of this ore resulted in an average 
copper content of 2.3 wt pet. To assess the potential copper removal, 
it is important to determine the extent and distribution of the cop- 
per in various minerals. This particular ore displayed the follow- 
ing copper distribution: 55 pet in chrysocolla, distributed mainly 
in fractures but also in altered plagioclase feldspar phenocrysts; 40 
pet in clay minerals, distributed in altered plagioclase feldspar and 
biotite phenocrysts; and 5 pet in limonite mineralization (1) (fig. 
9). Copper mobilization from these various minerals is dependent 
on both the accessibility of the ore minerals to the leach solution 
and the mineral solubility. The textural relationships between the 
copper-bearing minerals and the fractures in the rock will deter- 
mine the leach solution accessibility. Copper mineralization located 
along fractures (figs. 9-10) will have greater accessibility to the 



26 




2 6 1 y 



m 



Figure 9.— Left, backscattered electron image (BED of chrysocolla veinlet (center) crosscutting an altered biotite phenocryst in preleach sample 
of granodiorite porphyry ore. Right, BEI image with computer-enhanced X-ray overlay maps that show the distribution of single elements or 
combinations of elements: copper (red), iron (green), potassium (dark blue), and iron and potassium (light blue). Copper is distributed in this 
sample not only in chrysocolla but also in the altered biotite. The dark-blue area at the top of this map is potassium-feldspar. (These images 
were obtained from polished thin sections on the electron probe microanalyzer. The colorized X-ray dot maps were generated by rastering an 
electron beam across the sample annd counting the X-ray emissions of the selected elements with an energy-dispersive spectrometer.) 




Figure 10.— -Quartz monzonite crosscut by numerous factures filled by atacamite and clay mineralization; photomicrograph. 



^^H 



■H 



27 



leach solution than will disseminated copper mineralization (fig. 
11), especially if in the latter case the matrix permeability and frac- 
ture density are relatively low. Postleach petrologic examinations 
of areas that were contacted by the leach solution will also yield 
information on the relative solubilities of both the ore and gangue 
minerals. 

Three experiments were conducted on the chrysocolla ore using 
three different leach solution concentrations: 5, 15, and 25 g/L 
sulfuric acid (/). These experiments were conducted in the acrylic 
reaction cells with the leach solution delivered to the samples with 
a peristaltic pump (low-pressure system described earlier). Leachant 
was not recirculated during these experiments; thus, fresh leach 
solutions were continuously injected into the samples. The core 
samples were approximately 13 cm in length and had diameters 
near 5 cm. The experiments were conducted at ambient temperature 
and pressure conditions. 

Figures 12, 13, and 14 display the resulting fluid chemistry 
from experiments conducted on this ore at the three different con- 
centrations of sulfuric acid. When this information is combined with 
the results of the preleach and postleach petrologic and microprobe 
studies, conclusions can be drawn about the various reactions 
operating during leaching. For example, the graph of copper con- 
centration versus time (fig. 12) reveals that no copper was mobilized 
during the first 200 to 500 h of the experiment. However, ap- 
preciable amounts of calcium and sodium were mobilized during 
these initial stages of the experiments (figs. 13-14). X-ray diffrac- 
tion and microprobe studies of the clay minerals in this ore iden- 
tified significant calcium and sodium in the exchangeable sites of 
the clays. This, along with the elevated pH of the fluids during this 



time, led to the conclusion that no copper was mobilized in the in- 
itial period of the experiments because the acid was totally con- 
sumed by ion-exchange reactions taking place, which left no H + 
available for copper dissolution. After H + replaced all the ex- 
changeable cations in the clays, as evidenced by the rapid decrease 
in calcium and sodium in solution, copper dissolution proceeded. 
Thus, if a significant amount of clay mineralization is present in 
a deposit, there exists a potential for considerable acid consump- 
tion. The fact that there is an initial delay in copper production, 
yet high reagent consumption, may have a significant impact on 
the economic analysis of a deposit. Therefore, initial microprobe 
and leaching studies will help to quantify the significance of ion- 
exchange reactions for a particular ore. The Bureau is currently 
investigating various solutions that may be injected into the ore prior 
to acid injection to reduce or eliminate the impact of ion-exchange 
reactions on the leaching process. 

The leachant chemistry (figs. 13-14) and postleach petrologic 
analyses indicate that the biotite in this rock undergoes attack by 
the sulfuric acid leach solutions and contributes aluminum, iron, 
magnesium, and potassium to the leach solutions. While some 
aluminum and iron were removed from the clay minerals and 
limonite, respectively, biotite dissolution and the initial ion-exchange 
reactions in the clay minerals were the main mechanisms occurring 
in this rock that consumed acid. However, even though biotite 
dissolution was occurring, postleach microprobe studies revealed 
that only a minor amount of the sorbed copper present in the biotite 
(fig. 9) was removed. Most of the copper in solution was contributed 
by chrysocolla and altered plagioclase phenocrysts, with a minor 
amount from limonite (7). The implications of this are significant, 




»V # 



Figure 11.— Quartz monzonite containing disseminated atacamite mineralization; photomicrograph. 



28 



as previous studies have found that as much as 15 pet of the total 
copper in a sample is present in the biotite (7). Also, most of the 
copper contained in the clay minerals was solubilized during 
leaching, with little evidence of significant sorbed copper remain- 
ing in the clay minerals examined in the postleach samples. This 
again illustrates the value of the information obtained from the 
leaching and petrologic studies. 

In order to obtain the required information to perform an 
economic evaluation of in situ mining on a larger scale, it is 
necessary to determine the consumption of acid per unit of copper 
produced and the dependence of acid consumption on leach solu- 
tion concentration. For example, using the general equation for 
chrysocolla dissolution by sulfuric acid: 

CuSi0 3 -2H 2 + 2H + = Cu +2 + Si0 2 + 3H 2 0, 

the stoichiometric acid consumption is calculated to be 1.54 g 
sulfuric acid per 1 g of copper removed. By comparing the acid 
consumption observed in each experiment with the calculated 
stoichiometric value, one can evaluate the severity of gangue mineral 
acid consumption and draw correlations between consumption and 
leach solution concentration. As previously discussed, experiments 
performed on the granodiorite porphyry ore resulted in an initial 
very high consumption of acid due to ion-exchange reactions with 
the clay minerals (fig. 15), which decreased rapidly as the ion- 



exchange reactions neared completion. As copper mineralization 
becomes depleted, gangue mineral consumption of acid steadily in- 
creases, and it is proportional to acid concentration. Graphs such 
as figure 15 illustrate that core-leaching experiments can help to 
evaluate the reactivity of various types of ores and can also aid in 
determining the optimum leach solution concentration for specific 
ore types, as acid consumption will have to be weighed against cop- 
per loading in solution and copper recovery rate (1). 

The net permeability of the three samples increased during all 
three experiments, despite the fact that sulfate precipitation occurred. 
Sulfate concentrations in the leach solutions revealed that sulfate 
precipitation was occurring early in the experiments, before the con- 
centrations rebounded to the initial level of the fresh leach solu- 
tion. These depletions corresponded to the initial elevated calcium 
concentrations observed in the experiments. Therefore, it is likely 
that gypsum precipitation occurred during this time, as gypsum was 
observed in the postleach core samples. Also, the first few leachant 
samples contained gypsum that had precipitated from solution. 
However, the dissolution of chrysocolla had a stronger influence 
on permeability, resulting in the net increases in permeability for 
the three experiments. These increases ranged from 4 to 1 10 times 
the initial value, with the large increases resulting from the observed 
dissolution of fracture-hosted mineralization. It should also be noted 
that no chloride was present in the leachant because of the absence 
of chloride mineralization in this ore type. 



3 

u 



10,000 



8,000 



6,000 



4,000 



2,000 



KEY 
25 g/L H 2 S0 4 
15 g/L H 2 S0 4 
5 g/L H 2 S0< 




**> *b ^Oo '-tQo *'OQo <*b <4fc 3 '*Qo 

Time, h 

Figure 12.— Copper concentration in solution versus time for three core-leaching experriments using granodiorite porphyry-hosted chrysocolla ore. 



29 




500 1,000 1,500 2,000 2,500 3,000 
Time, h 



Figure 13.— Aluminum, calcium, and iron concentration in solution versus time for three core- 
leaching experiments using granodiorite porphyry-hosted chrysocolla ore. 



30 



Dim 
400 


i ■ ■ — i — 


5 


g/L 


i 

H 2 S0 4 


T 


1 

KEY 

Na 


300 












Mg 
K 


200 












- 


100 


-^—- i i 











- 


n 









500 






DO 
2 




500 1,000 1,500 2,000 2,500 3,000 
Time, h 

Figure 14.— Magnesium, potassium, and sodium concentration in solution versus time for 
three core-leaching experiments using granodiorite porphyry-hosted chrysocolla ore. 



31 




4oo *Qo 7 <9ao r '<kb *'<>ao **b *'*b 3 '*Qo 

Time, h 



Figure 15. — Acid consumption versus time for three core-leaching experiments using granodiorite porphyry-hosted chrysocolla ore. 



Atacamite Hosted in Quartz Monzonite 

A second type of ore examined was a quartz monzonite, which 
contained atacamite as the predominant copper mineral. The rock 
is medium to coarse grained and exhibits a relatively low matrix 
permeability. Petrologic studies of preleach thin sections determined 
that the average modal mineralogy of this ore was 40 pet quartz, 
25 pet potassium-feldspar, 20 pet sericite, 5 pet atacamite, 5 pet 
limonite, and 5 pet clay mineralization. The copper content of these 
samples ranged from 3 to 9 wt pet, with the large variability 
attributable to the substantial fracture density variations from sam- 
ple to sample. Unlike the granodiorite porphyry -hosted chrysocolla 
ore previously discussed, nearly all of the copper in this rock is 
located in fracture-hosted atacamite mineralization (figs. 10, 16), 
with minor amounts distributed in the clay minerals and limonite 
(7). These differences in mineralogy and texture result in very dif- 
ferent leaching characteristics from those displayed in the 
chrysocolla ore. 

Three experiments were conducted on this ore using three dif- 
ferent leach solution concentrations: 10, 20, and 40 g/L sulfuric 
acid (/). These experiments were also conducted in the acrylic reac- 
tion cells, with the leach solution delivered to the samples with a 
peristaltic pump. Leachant was not recirculated during these ex- 
periments; thus, fresh leach solutions were continuously injected 
into the samples. The samples used in these experiments were split 
pieces of 5-cm-diameter core approximately 6 cm long. These 



experiments were also conducted at ambient temperature and 
pressure conditions. 

Because of the high ore grade, the distribution of nearly all 
ore mineralization in fractures, and the relative absence of acid- 
consuming gangue minerals, copper loadings in solution were very 
high in these experiments (fig. 17). The absence of acid-consuming 
gangue mineralization is indicated by the fluid chemistry from the 
experiments (figs. 18-19) and was also supported by petrologic 
studies. Only a small number of ion-exchange reactions with clays 
occurred, as shown by the rapid decrease in calcium, potassium, 
and sodium in solution. A relatively small amount of iron was mobi- 
lized as the result of limonite dissolution. The absence of biotite 
in these samples resulted in virtually no magnesium mobilization. 

The low reactivity of the gangue minerals in these samples is 
reflected in the graph of unit acid consumption versus time for the 
experiments (fig. 20). The average consumption for the three ex- 
periments ranged from 1 .2 to 1 .5 g sulfuric acid per gram of cop- 
per. In the dissolution of atacamite, 

Cu 2 (OH) 3 Cl + 4H + = 2Cu +2 + HC1 + 3H 2 0, 

it is seen that hydrochloric acid is generated. Thus, 1.5 mol of 
sulfuric acid (3 H + ) is consumed in the production of 2 mol of cop- 
per, resulting in an acid consumption of 1.16 g sulfuric acid per 
1 g of copper removed. Thus, the acid consumption observed in 
these experiments is very close to this stoichiometric consumption. 



32 




1 6 3 



r 



m 



Figure 16. — Left, backscattered electron image (BED of fracture-hosted atacamite mineralization (brightest areas) in preleach sample of quartz 
monzonite ore. Right, BEI image with computer-enhanced X-ray overlay maps that show the distribution of copper (red), iron (green), and silicon 
(blue). Copper is distributed almost entirely in fractures. The dark-blue areas correspond with quartz, and the green areas correspond with limonite. 



40,000 



10,000 



5,000 



1,000 



KEY 
40 g/L H 2 S0 4 
20 g/L H 2 S0 4 
10 g/L H 2 S0 4 




3,000 



4,000 



2,000 
Time, h 

Figure 17. — Copper concentration in solution versus time for three core-leaching experiments using quartz monzonite-hosted atacamite ore. 



33 



300 



200 



100 





10 g/L H 2 S0 4 


— i — — — 

KEY 


■ 




Fe 






Al 
Mg 


V 







300 



GO 



a* 



200 



100 




4,000 



Figure 18. — Aluminum, iron, and magnesium concentration in solution versus time for three 
core-leaching experiments using quartz monzonite- hosted atacamite ore. 



34 



uuu 
900 


r ■■ 1 1 




800 


10 g/L H 2 S0 4 


KEY 


700 


- 


Ca 


600 


\ 


K 


500 


i 


^la~ 


400 


1 


- 


300 


\ 


- 


200 




- 


100 
n 








4,000 



Figure 19.— Calcium, potassium, and sodium concentration in solution versus time for three 
core-leaching experiments using quartz monzonite- hosted atacamite ore. 



35 



u 

i_ 

Q. 
60 

c 
o 

»mm 

E 

3 
«o 

C 

8 

• MM 



1 



KEY 
40 g/L H 2 S0 4 
20 gft. H 2 S0 4 
10 g/L H 2 S0 4 




1,000 



2,000 
Time, h 



3,000 



4,000 



Figure 20. — Acid consumption versus time for three core-leaching experiments using quartz monzonite-hosted atacamite ore. 



An important observation is that the acid consumption in these three 
experiments was relatively independent of acid concentration, unlike 
acid consumption with the granodiorite porphyry ores. Therefore, 
a higher acid concentration can be used to achieve greater copper 
loadings in solution and a greater recovery rate without a prohibitive 
increase in acid consumption. 

As in the experiments conducted on the chrysocolla ore, 
permeability also increased in all three of the atacamite experiments. 
The initial permeabilities of these samples were very low, and nearly 
all copper mineralization was fracture hosted, which led to extremely 
large increases in permeability relative to the initial values as 
atacamite was leached from the extensive fracture network. 
Permeability increased from 100 to 1,300 times in these samples 
as leaching progressed. It was initially very difficult to inject the 
leach solution into the samples, but eventually the flow rate increased 
as atacamite was removed from the fractures. Thus, in situ mining 
may be applicable to ores that have low initial permeabilities, 
because mineral dissolution increases the permeability. A small 
amount of gypsum was observed on the sericite phenocrysts in the 
postleach samples. However, little S0 4 " 2 depletion was observed 
in the leachant, indicating that sulfate precipitation was relatively 
minor in these experiments. The molar ratio of copper to chlorine 
was near 2 throughout the experiment, indicating that very little, 
if any, chloride precipitated during leaching. 



Applicability of Laboratory Experiments to Deposit 
Evaluations 

Conducting a few core-leaching experiments and combining 
leaching chemistry data with preleach and postleach petrologic 
studies of the samples can provide a great deal of information about 
the applicability of in situ mining for a particular type of ore. Prior 
to initiating an in situ mining operation, both the types and abun- 
dances of gangue minerals present in the various ore types should 
be identified, as well as their reactivities with the leach solution. 
This identification will also assist in optimizing the leach solution 
concentration and will help identify and control potentially detrimen- 
tal secondary reactions that may occur during leaching. For exam- 
ple, clay minerals can consume large amounts of acid in the ex- 
change of calcium, resulting in gypsum precipitation. Therefore, 
it may be necessary to treat the ore body with a solution prior to 
leaching with acid in order to remove the exchangeable clay ions 
that can have deleterious effects on in situ mining. Alternatively, 
it may be necessary to avoid leaching certain parts of the ore body 
in order to eliminate problems that may affect a larger portion of 
the well field. It is also important to determine the spatial and mineral 
distribution of copper in the ores. The solubilities of the various 
copper-bearing phases and the accessibility of leach solution to them 
will control the total copper recovery and recovery rates. The six 



36 



experiments described above resulted in copper recoveries rang- 
ing from 57 to 90 pet; leach solution accessibility was the major 
limiting factor influencing copper recovery. Although copper 
recovery achieved in the laboratory may be different from that ob- 



tained in the field because of scale differences, relatively high 
recoveries can be attained if an adequate portion of the ore minerals 
can be contacted by the leach solution. 



SUMMARY 



The Bureau has developed a comprehensive laboratory research 
program to study both the chemical and physical influences on in 
situ mining. The three main areas of this program are core-leaching 
experiments, monitoring the resulting fluid chemistry from these 
experiments, and preleach and postleach petrologic studies. Core- 
leaching experiments more closely simulate the nature of the 
solution-ore mineral contact in situ than do experiments with crushed 
rock. Three core-leaching systems are currently being used to con- 
duct experiments on oxide copper ores. The basic design is similar 
in all three systems; the main differences are in the materials used 
to construct the reaction cells and the pumps used to deliver the 
leach solution to the samples. The necessary features of each type 
of system include flexibility to accommodate a variety of sample 
sizes and shapes, nonreactivity with the leach solution, relatively 
low cost, and ability to recover postleach ore samples for petrologic 
studies. These systems cover the pressure range of to 1,000 psi 
and flow rates of 2 mL to several liters per day. These systems 
can also be utilized to conduct leach solution recirculation 
experiments to study the effects of ion accumulations in the leach 
solution on the leaching chemistry during in situ mining. 

The significance of mineralogy and texture on leaching 
characteristics was evident when results from experiments using 



two distinctly different ore types were compared. A granodiorite 
porphyry that contained fracture-hosted chrysocolla and 
disseminated copper in altered plagioclase phenocrysts contained 
two gangue minerals that contributed significantly to acid consump- 
tion. Very high initial acid consumption was due to the exchange 
of H + for Ca +2 and Na + in the clay minerals. Copper mobilization 
did not commence until nearly all of the exchangeable cations had 
been depleted from the clay minerals. The other gangue mineral, 
biotite, showed appreciable attack by the sulfuric acid leach solu- 
tions, resulting in the mobilization of aluminum, iron, magnesium, 
and potassium. However, postleach petrologic studies determined 
that little of the copper contained in the biotite was solubilized. Acid 
consumption was proportional to leach solution concentration. The 
second type of ore studied was a quartz monzonite porphyry that 
contained all copper mineralization as fracture-hosted atacamite. 
This ore was relatively free of acid-consuming gangue minerals such 
as biotite and clay minerals, resulting in maximum acid consump- 
tions between 1.2 and 1.5 g sulfuric acid per gram of copper. In- 
formation such as this is necessary to properly evaluate various ore 
types for their suitability for in situ mining. 



REFERENCES 



1. Paulson, S. E. Core Leaching Experiments To Assess Leaching 
Characteristics During In Situ Mining of Oxide Copper Ores. Paper in 
Proceedings, In Situ Recovery of Minerals (Eng. Found. Conf., Santa 
Barbara, CA, Oct. 25-29, 1987). AIME, in press. 

2. Paulson, S. E., L. J. Dahl, and H. L. Kuhlman. In Situ Mining 
Geologic Characterization Studies: Experimental Design, Apparatus, and 
Preliminary Results. Miner. & Metall. Process., v. 4, No. 4, 1987, pp. 
181-189. 

3. Larson, W. C, J. K. Ahlness, and S. E. Paulson. The Bureau of Mines' 
Role in the Development of True In Situ Copper Mining as a Future 
Technology. Miner. Resour. Eng., v. 1, No. 2, 1988, pp. 171-180. 

4. Cook, S. S., and S. E. Paulson. Leaching Characteristics of Selected 
Supergene Copper Ores. Min. Eng. (Littleton, CO), v. 41, No. 1, 1989, 
pp. 33-39. 



5. Lewis, W. E., and S. Tandanand (eds.). Bureau of Mines Test 
Procedures for Rocks. BuMines IC 8628, 1974, 223 pp. 

6. Davidson, D. H., R. V. Huff, R. E. Weeks, and J. F. Edwards. 
Generic In Situ Copper Mine Design Manual (contract J0267001, Science 
Applications Int. Corp.). Volume II: Draft Generic In Situ Copper Mine 
Design Manual, 1988, 454 pp.; for inf. contact J. K. Ahlness, TPO, Twin 
Cities Res. Cent., BuMines, Minneapolis, MN. 

7. Cook, S. S. Petrologic Analysis of Laboratory Core Leaching 
Experiments. Paper in Proceedings, In Situ Recovery of Minerals (Eng. 
Found. Conf., Santa Barbara, CA, Oct. 25-29, 1987). AIME, in press. 



37 



METHODS FOR DETERMINING THE GEOLOGIC STRUCTURE OF AN ORE 
BODY AS IT RELATES TO IN SITU MINING 

By Linda J. Dahl 1 



ABSTRACT 

As part of its in situ mining research program, the U.S. Bureau of Mines is studying the 
geologic structure of the Santa Cruz and Casa Grande West porphyry copper deposits near 
Casa Grande, AZ. A datum joint set technique was developed to measure joint orientations 
on unoriented drill core. Based on regional fabric and oriented-core data, a particular joint 
set is defined as the datum joint set that is oriented in a certain direction. Other fracture 
orientations are measured relative to this datum joint set. The major joint sets are indentified 
by plotting lower hemisphere Schmidt equal-area projections. This technique allows the 
structural data base of a deposit to be extended while keeping the high cost of oriented-core 
drilling to a minimum. Methods of calculating fracture frequency and fracture density are 
also discussed. 

The major joint orientations at the Santa Cruz and Casa Grande West deposits strike 
northeast and northwest. The most prominent joint set strikes northeast and dips 70° to the 
northwest with a frequency of 0.76 to 0.98 fracture per foot. The frequency of other iden- 
tified joint sets ranges from 0.19 to 0.41 fracture per foot. 



INTRODUCTION 



A thorough study of the geology of an ore body is impor- 
tant in planning and operating underground, open pit, or in 
situ mining operations. Exploratory drilling provides much of 
the information necessary to evaluate a deposit for its mining 
potential. The principal uses of drilling data at conventional 
mining operations are to calculate ore reserves, predict ground 
support requirements, minimize the amount of bad ground 
through which headings must be driven, and minimize water 
inflow to the mine. Drilling data are also important for plan- 
ning an in situ mining operation. Since fractures are the 
primary fluid flow channels during in situ mining, determining 
the primary joint patterns and preferred orientation of frac- 
tures containing ore minerals is important. Previous studies 
suggest that a preferred orientation of flow is related to 
primary fracture orientations (I). 2 This information can be 



useful in designing in situ mining well patterns. Modifying well 
field design (e.g., geometry and spacing) to take full advantage 
of preferred flow direction and designing injection wells that 
intersect an optimum number of mineralized fractures may 
result in increased metal recoveries. Although this has not yet 
been demonstrated for in situ mining, it will be investigated in 
future experiments. 

The geologic structure of an ore deposit is often deter- 
mined with the aid of oriented drill core. However, drilling 
costs are approximately doubled when drilling oriented core. 
Therefore, the use of oriented core is usually kept to a 
minimum. Thus, as part of its in situ mining research program, 
the Bureau of Mines developed a technique to extend the struc- 
tural data base of an ore deposit at a low cost relative to drill- 
ing oriented core, by using datum joint sets. 



BACKGROUND 



The Bureau is conducting an in situ mining field research 
project at the Santa Cruz and Casa Grande West porphyry 
copper deposits near Casa Grande, AZ (figs. 1-2). The Santa 



'Geologist, Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, 
MN. 

2 Italic numbers in parentheses refer to items in the list of references at the end 
of this paper. 



Cruz deposit is owned by ASARCO Santa Cruz Inc. and 
Freeport Copper Co. The Casa Grande West deposit is owned 
by Casa Grande Copper Co. The general geology and structure 
of the Santa Cruz deposit are being studied using exploration 
drill core. The structural geology of the Casa Grande West 
deposit has been well documented with oriented drill core (2). 
To date, there has been no oriented core drilled in the Santa 
Cruz portion of the deposit. 



38 




Figure 1.— Map location of Santa Cruz and Casa Grande West ore deposits and surrounding 
mountains where outcrop data were obtained. 





Sec. 13 



SC-19A 

Santa Cruz 
deposit 

(ASARCO-Freeport) 

:?:■. * ■ 
SC-46A 



ir 



Property line 



Casa Grande West 
deposit 

: (Casa Grande Copper Co.) 




Sec. 18 



Clayton Rd. 



1 1 A mile 



Township 6 South 
Range 4 East 
Pinal County, AZ 



Figure 2.— Santa Cruz and Casa Grande West deposits. 



39 



As part of the in situ mining field test program, two new 
holes were wedged off two previously drilled exploration holes 
(SC-19 and SC-46) in the Santa Cruz deposit. Unoriented core 
was obtained near the top of the ore zone in SC-19A (222 ft, 
from 1,239- to 1,461-ft depth) and SC-46A (275 ft, from 1,410- 
to 1,685-ft depth). 



Holes SC-19A and SC-46A were drilled in a supergene 
oxide ore deposit. The primary lithologic units are Oracle 
Granite, quartz monzonite porphyry, and infrequent diabase 
dikes. Supergene copper mineralization includes chrysocolla, 
atacamite, and copper clays. Copper mineralization occurs 
primarily as fracture fillings but also is disseminated. 



DATUM JOINT SET TECHNIQUE 



HYPOTHESIS 

Based on Heidrick's (5) study of porphyry copper deposits 
in the Southwestern United States and oriented core data from 
the Casa Grande West deposit (2), areas that have northeast- 
trending joints dipping to the northwest, it is hypothesized that 
the steeply dipping joints observed in the Santa Cruz core 
samples also trend northeast and dip to the northwest. As will 
be discussed later, this assumption is fully supported by field 
measurements acquired near the deposit. 



PROCEDURE 

A joint set dipping 70° to 80° is present in the core. This 
steeply dipping joint set is hypothesized to strike northeast and 
dip northwest and is defined as the datum (reference) joint set. 
At each datum joint, as much surrounding core as possible was 
reassembled and, where necessary, taped together with mask- 
ing tape (fig. 3). The drill hole was assumed to be vertical, and 
dip angles were measured and marked next to each fracture. A 
mark was made on the datum joint indicating the dip direc- 
tion. The length of reassembled core was placed in a core tray 
with the dip direction of the datum joint aligned with the edge 
of the tray. All the other pieces of core were fit together on 
either side of the piece containing the datum joint to align 
them in their original orientation relative to the datum joint. 



The core tray edge was used as a guide to mark an "orientation 
line" along the length of core (fig. 3). 

A goniometer is used to measure joint orientations on 
oriented drill core. Since the core used in this study was 
unoriented, it was necessary to modify the standard 
goniometer measuring technique. The goniometer was set at 
the north 45° east orientation, which is an average orientation 
of the joints in the area. Core was placed in the goniometer 
with the orientation line on the core aligned with the 
goniometer orientation line, ensuring that the primary joint set 
was dipping northwest (fig. 4). Without moving the core, the 
goniometer was then rotated to measure the relative strikes 
and dips of all other fractures (fig. 5). 

Many pieces of core were just a few inches in length. To 
take measurements on these small pieces of core, a 25-cm-long 
clear plastic extension tube was fabricated and used with wood 
(25-cm length) and polyvinyl chloride (PVC) spacers (5-, 7-, 
and 17-cm lengths) to raise and support various sizes of core to 
the proper height. 

ANALYTICAL PRECISION 

Dip angles were measured to the nearest 5°. Strikes of 
70°- to 80°-dipping joints that were not identified as datum 
joints had relative measurements, which varied from the 
datum joint strike by approximately ±15°. Precision in 
goniometer measurements was approximately ±5°. 








Figure 3.— Marking orientation line on reassembled core. 



40 




Figure 4.— Aligning datum joint set orientation line on core with goniometer orientation line. 



41 




Figure 5.— Measuring relative orientation of fracture. 



42 



PRIMARY JOINT ORIENTATIONS 



A total of 752 fractures, 383 from hole SC-19A and 369 
from hole SC-46A, were measured. The joint orientation data 
for both holes were plotted on lower hemisphere Schmidt 
equal-area projections (4). These plots (fig. 6) show ortho- 
gonal joint patterns, with the primary joints striking northeast 
and northwest. Northeast-trending joints dip to the northwest 
and southeast, and northwest-trending joints dip to the north- 
east and southwest. 

Strike-frequency rosette diagrams were prepared for all 
fractures (fig. 7), mineralized fractures (fig. 8), and un- 



mineralized fractures (fig. 9). There was no significant dif- 
ference seen between the strikes of mineralized and un- 
mineralized fractures. Approximately 25 pet of all fractures 
measured contained copper mineralization (atacamite and 
chrysocolla). The strikes of approximately 60 pet of fractures 
containing copper mineralization were plotted in the northeast 
quadrant, and 40 pet of the strikes were plotted in the north- 
west quadrant. Copper mineralization was observed to be 
most prevalent in steeply dipping fractures and many near- 
vertical fractures (fig. 10). 





>3% 





KEY 






1%-2% 


I 


] 3%-4% 




2%-3% 




1 >4% 



Figure 6.— Lower hemisphere Schmidt equal-area projections for Santa Cruz joints using datum joint set technique. Percen- 
tages indicate the number of data points that fall within a 1-pct area of the circle. 



43 




20% 15% 10% 5% 



SC-19A 





E W 

5% 10% 15% 20% 20% 15% 10% 5% 5% 10% 15% 20% 

SC-46A 



Figure 7.— Strike-frequency rosettes for all fractures, holes SC-19A and SC-46A. 




20% 15% 10% 5% 5% 10% 15% 20% 20% 15% 10% 5% 5% 10% 15% 20% 
SC-19A SC-46A 

Figure 8.— Strike-frequency rosettes for mineralized fractures, holes SC-19A and SC-46A. 





20% 15% 10% 5% 5% 10% 15% 20% 20% 15% 10% 5% 5% 10% 15% 20% 

SC-19A SC-46A 

Figure 9.— Strike-frequency rosettes for unmineralized fractures, holes SC-19A and SC-46A. 



44 



SC-19A 
N 




Figure 10.— Lower hemisphere Schmidt equal-area projections for Santa Cruz mineralized fractures. Percentages indicate the 
number of data points that fall within a 1-pct area of the circle. 



VERIFICATION OF DATUM JOINT SET HYPOTHESIS 



The joint patterns of the Casa Grande West deposit are 
well documented from previous oriented-core drilling (2). A 
composite lower hemisphere Schmidt equal-area projection 
showing the Casa Grande West joint sets (2) is shown in figure 
11. The four predominant joint sets are (1) a northeast- 
trending set that dips approximately 50° to the southeast, (2) a 
northeast-trending set that dips approximately 45° to the 
northwest, (3) a northwest-trending set that dips approximate- 
ly 45° to the northeast, and (4) a northwest-trending set that 
dips approximately 40° to the southwest. White (2) did not 
report many steeply dipping fractures, postulating that this 
was due to the undersampling of near-vertical joints by a ver- 
tical drill hole. This undersampling will be discussed in detail 
in the next section, "Fracture Frequency." 



Table 1 is a comparison of joints identified using oriented 
core in the Casa Grande West deposit with the joints identified 
using the datum joint set technique at the adjacent Santa Cruz 
deposit. The values are averages for all measurements re- 
corded. There is good correlation between the data obtained 

Table 1.— Comparison of joint orientation data (average strike- 
dip) from Santa Cruz and Casa Grande West ore deposits 

Deposit NE-NW NE-SE NW-NE NW-SW 

Casa Grande West (2) 45° 50° 45° 40° 

Santa Cruz: 

SC-19A 70° 45° 50° 50° 

SC-46A 70° 45° 45° 45° 



45 



using oriented core and the datum joint set technique. The 
primary difference observed was that Santa Cruz joints trend- 
ing northeast and dipping northwest are somewhat steeper 
than those measured at Casa Grande West. One possible ex- 
planation for this difference is that the data collected at the 
Casa Grande West deposit were from greater depths (approx- 
imately 1,800 to 2,950 ft deep) than at the Santa Cruz deposit 
(approximately 1,240 to 1,685 ft deep). Many of the faults and 
fractures in this area are observed to be listric in shape, which 
could account for more shallowly dipping fractures at greater 
depths. 

To further verify the structural fabric of the Santa Cruz 
deposit, joint orientations were measured at nearby outcrops 
of the Oracle Granite, which is the same rock unit that hosts 
the ore deposit. The nearest outcrops of the Oracle Granite are 
located in the Sacaton Mountains to the north of the deposit, 
Table Top Mountains to the west, and Silver Reef Mountains 
to the southeast (fig. 1). Over 700 joints were measured on out- 
crops surrounding the Santa Cruz Basin, using a Breithaupt 
Kassel 3 geological stratum compass. As seen in the lower 
hemisphere Schmidt equal-area projections developed from 
the outcrop data (fig. 12), joint orientations are similar to 
those observed in the core samples from the Santa Cruz and 
Casa Grande West deposits. These field measurements verify 
that the primary joints are steeply dipping and trending north- 
east and northwest. 




'Reference to specific products does not imply endorsement by the Bureau of 
Mines. 



Figure 11.— Lower hemisphere Schmidt equal-area projec- 
tion for Casa Grande West joints. Contour numbers are 
percentages that indicate the number of data points that fall 
within a 1-pct area of the circle. (Based on data from White (2).) 




Silver Reef 
N 



Table Top 
N 






KEY 


j/i£s 




KEY 




r c * ■* 


1%-4% 




10%-1 3% 




1%-2% 


; 


] 3%-4% 
















| >4% 




4%-7% | >13% 




2%-3% 














:■■■■"■■'■ : ..\ 


7%-10% 







rHP^v\ 






KEY 


M 




1%-3% [ 




5%-7% 






>7% 


3%-5% 



Figure 12.— Lower hemisphere Schmidt equal-area projections for Sacaton, Silver Reef, and Table Top Mountains. Percentages 
indicate the number of data points that fall within a 1-pct area of the circle. 



46 



FRACTURE FREQUENCY 



A common technique used to estimate fracture frequency 
is simply to count the number of fractures intersected in a drill 
hole and divide it by the drill-hole length. However, these 
estimates are dependent on the orientation of the observation 
line relative to the joints. In many porphyry copper systems in 
the Southwestern United States, the strikes of fractures are not 
random (J), and a change in observation line can cause large 
changes in observed fracture frequencies. Thus, there is a 
natural bias against intersecting joints as the angle between the 
joints and the observation line decreases. The actual fracture 
frequency is greatest in a line normal to the joint set (fig. 13). 
Therefore, fracture frequencies based on the number of frac- 
tures intersected in a drill hole are misleading. The following 
calculations indicate that actual fracture frequencies may be 
underestimated by as much as two-thirds by the direct observa- 
tion method. Observed fracture frequency can be converted to 
actual fracture frequency (number of fractures encountered 
along a line normal to the dip of the joint set) using the follow- 
ing equation: 

Actual fracture frequency = (N d /L d )/cos 9, (1) 

where N d = number of fractures observed in a drill hole, 

L d = length of drill hole, 
and 6 = dip of joint sets. 



Actual fracture frequencies were calculated for each joint 
set, using the data obtained with the datum joint set technique 
(table 2). The effect of observation line bias is most 
dramatically seen in the steeply dipping joint set (striking 
northeast and dipping 70° northwest), where the observed 
fracture frequency is only a third of the actual fracture fre- 
quency. 



Table 2.— Fracture frequencies 



Hole 



SC-19A 



SC-46A 



Orientation 


Fracture 


per foot 


Strike 


Dip 


Observed 


Actua 


NE 


45° SE 


0.29 


0.41 


NE 


70° NW 


.35 


.98 


NW 


50° SW 


.22 


.34 


NW 


50° NE 


.20 


.30 


NE 


45° SE 


.23 


.34 


NE 


70° NW 


.27 


.76 


NW 


45° SW 


.13 


.19 


NW 


45° NE 


.20 


.30 



Normal to 
joint set 




Joint set 



Drill hole 



KEY 
L(j Length of drill hole 
© Joint set dip 



Figure 13.— Undersampling near-vertical joints in a vertical drill hole. 



47 



FRACTURE DENSITY 



Fracture frequencies calculated with a count of the 
number of joints in a particular set that intersect a borehole 
divided by the total borehole length (even those corrected for 
observation line bias) do not adequately describe possible 
fluid-carrying surfaces. Fracture frequencies calculated in this 
way reflect only one set of fractures and not the influence of 
all the fractures at particular depths. Another way of 
calculating fracture frequency in drill core is by utilizing the 
fracture surface area per core volume. Calculations using frac- 
ture surface area per core volume will be referred to as fracture 
densities to differentiate them from fracture frequencies 
calculated using simple counts of fractures. 

Using drill core, fracture densities can be calculated using 
the quotient of the fracture surface area (7rrVsin f ) and the 
volume of length of core (7rr 2 L) (fig. 14) (J). The integrated 
densities are the sum of these quotients: 



where 



f=i 



7rr 2 /sin f 
irr 2 L 



E 

f=i 



1 



(sin f )L 



(2) 



and 



n = 

f = 
t = 
r = 

f = 

L = 



fracture density, 

each individual fracture, 

total number of fractures, 

radius of drill core, 

angle fracture makes with long axis of core, 

length of drill core. 



This is a simple calculation using only the fracture angle with 
reference to the long axis of the drill core and the total length 
of core. A fracture density plot to overlay geologic cross sec- 
tions can be generated and contoured to give an indication of 
highly fractured zones, which may be potential high-flow 
areas. 

Nearly 11,000 fractures were recently measured on drill 
core to obtain fracture densities for 6 drill holes in the Santa 
Cruz deposit. Data are currently being compiled, and potential 
high-flow areas are being contoured. A field test to correlate 
actual field permeability measurements with those predicted 
using this method is scheduled for this fiscal year (1989). 






fracture area 




core volume 


_ 


7T r 2 / sin fy 




77-r 2 L 


= 


1 



( sin f ) L 

where f = each individual fracture, 
n = fracture density, 
r = radius of drill core, 
L = length of drill core, 

and 9t = fracture angle. 



Figure 14.— Fracture density using fracture area per core volume. (Based on data from Haynes (5).) 



48 



PERMEABILITY DETERMINATIONS USING FRACTURE DENSITIES 



One of the variables controlling fluid flow rates is flow 
porosity or permeability (6). Norton (7) suggests that 
permeability of a fractured plutonic rock may be approx- 
imated by a simple flow porosity model. From Darcy's law for 
tluid flow through a porous medium, it can be shown that for 
fractured rocks, permeability is a function of both the abun- 
dance of fractures and the cube of the fracture aperture or 
width (5). 

k.-f- (3) 

where k = permeability, cm 2 , 

n = fracture density, cm -1 , 
and d = fracture aperture, cm. 

Measuring fracture apertures accurately is difficult. Using ac- 
tual field-measured permeabilities from holes SC-19A and SC- 



46A, fracture apertures were calculated using equation 3 to be 
7.5 and 4.3 /*m, respectively. Since these calculated fracture 
aperture numbers are very small ( < 10 /mi), they have little ef- 
fect on equation 3. If the fracture aperture (d) is held constant, 
permeability calculated using equation 3 is most greatly in- 
fluenced by fracture density. 

Since fractures are the primary fluid flow channels during 
in situ mining, maximizing the number of mineralized frac- 
tures contacted by the leach solutions can result in increased 
recoveries. Increasing the number of fractures a drill hole in- 
tersects increases observed fracture density and therefore 
permeability. Two of the ways this can be accomplished are by 
hydrofracturing and by drilling directional holes. Both tech- 
niques are costly, but the reward of higher total recovery, 
greater flow rates, and increased control over solution flow 
could potentially justify the added expense. 



SUMMARY 



The Santa Cruz deposit is a highly fractured deposit with 
primary joints striking northeast and northwest. Orthogonal 
fracture patterns are observed with northeast-trending frac- 
tures dipping northwest and southeast and northwest-trending 
fractures dipping northeast and southwest. Copper mineraliza- 
tion is most prevalent in steeply dipping, northeast-trending 
fractures. 

The results obtained using the datum joint set hypothesis 
were consistent with the major joint sets measured using 
oriented drill core on the Casa Grande West deposit. The 
results were also consistent with the regional fabric measured 
on outcrops surrounding the Santa Cruz Basin. The or- 
thogonal joint patterns observed in the Santa Cruz deposit are 
also typical of porphyry copper deposits in the Southwestern 
United States as described by Heidrick (3). The technique of 



using a datum joint set can be effective in extending the struc- 
tural data base of a deposit without adding the prohibitive 
costs involved with extensive oriented-core drilling. 

Fracture frequencies based on the number of fractures in- 
tersected in a drill hole are misleading. Actual fracture fre- 
quencies may be underestimated by as much as two-thirds by 
the direct observation method. Fracture frequencies are most 
accurately described along a line normal to the joint set and 
can be corrected using a simple calculation. 

Fracture frequencies do not adequately describe possible 
fluid-carrying surfaces for qualitatively estimating permeabili- 
ty. Fracture densities calculated using fracture surface area per 
core volume reflect the effect of all joint sets and allow con- 
touring of potential high-flow areas. 



REFERENCES 



1. Larson, W. C, and T. H. McCormick. Utilizing Geologic 
Characterization Techniques To Evaluate an Unsaturated Gold 
Deposit for In Situ Mining. Paper in Application of Rock 
Characterization Techniques in Mine Design, ed. by M. Karmis (Proc. 
Symp., New Orleans, LA). AIME, 1986, ch. 10, pp. 88-97. 

2. White, D. H. Rock Mass Characterization and Preliminary Mine 
Design Estimates. Casa Grande West Deposit. Casa Grande Copper 
Co. internal rep., Feb. 1981, 114 pp.; available from L. Dahl, Twin 
Cities Res. Cent., BuMines, Minneapolis, MN. 

3. Heidrick, T. L., and S. R. Titley. Fracture and Dike Patterns in 
Laramide Plutons and Their Structural and Tectonic Implications. 
Ch. in Advances in Geology of the Porphyry Copper Deposits. 
Southwestern North America, ed. by S. R. Titley. Univ. AZ Press, 
1982, pp. 73-91. 



4. Billings, M. P. Structural Geology. Prentice-Hall, 1964, pp. 
107-115. 

5. Haynes, F. M. Vein Densities in Drill Core, Sierrita Porphyry 
Copper Deposit, Pima County, Arizona. Econ. Geol. and Bull. Soc. 
Econ. Geol., v. 79, 1984, pp. 755-758. 

6. Haynes, F. M., and S. R. Titley, The Evolution of Fracture- 
Related Permeability Within the Ruby Star Granodiorite, Sierrita 
Porphyry Copper Deposit, Pima County, Arizona. Econ. Geol. and 
Bull. Soc. Econ. Geol., v. 75, 1980, pp. 673-683. 

7. Norton, D., and R. Knapp. Transport Phenomena in 
Hydrothermal Systems: The Nature of Porosity. Am. J. Sci., v. 277, 
1977, pp. 937-981. 



49 



COMPUTER MODELING APPLICATIONS IN THE CHARACTERIZATION 

OF IN SITU LEACH GEOCHEMISTRY 



By Dianne C. Marozas 1 



ABSTRACT 

This paper discusses U.S. Bureau of Mines research on using computerized models to 
characterize in situ mining. Analytical results from Bureau leaching tests were input into the 
U.S. Geological Survey (USGS) computer program NEWPHRQ, which calculated the com- 
ponent speciation in solution and the stability of mineral phases with respect to the leach 
solutions. Data from this type of analysis can be used to predict ore solubilities, gangue 
mineral interferences, and metal mobilities under in situ leach mining conditions. One of the 
most powerful uses of computer modeling programs, such as NEWPHRQ, is to follow 
hypothetical reaction paths defined by the user and to calculate the solution compositions 
that result. Several hypothetical models are presented that demonstrate the potential applica- 
tions of reaction path modeling to in situ mining. With additional laboratory data on 
dissolution rates and metastable phase equilibrium, this type of analysis can be used to 
calculate the metal recoveries expected for a variety of mining situations. 



INTRODUCTION 



Computer models can be valuable tools for predicting the 
behavior of complicated in situ leach mining systems, where a 
large number of variable physical and chemical parameters can 
affect the efficiency of metal recovery. Bureau of Mines 
laboratory experiments in leaching whole-core copper oxide 
ore have provided the preliminary data needed to initiate 
modeling efforts in the characterization of the chemistry of 
leaching solutions and in projecting the reaction paths of in 
situ chemical systems. However, individual laboratory ex- 
periments are constrained by the unique variables operating 
under the conditions of the experiment. Geochemical com- 
puter models can extend the predictive usefulness of 



laboratory studies by numerically simulating the effects that 
changing variables will have on leaching results. 

The objective of this paper is to demonstrate the range of 
potential applications of geochemical modeling for practical 
use in in situ leach mining programs by presenting results from 
several hypothetical models. The models are designed to test 
the relative variation in projected metal recovery under a varie- 
ty of conditions that might be encountered in the field. An ac- 
curate computer model for characterizing in situ phenomena is 
a more efficient way to estimate the effects of a wide number 
of variables on metal recovery than time-consuming and costly 
laboratory and field investigations. 



BACKGROUND 



Computers perform numerical simulations of in situ 
leaching by solving the set of mathematical equations that de- 
scribe chemical interactions in a system at equilibrium. 
Chemical equilibrium models can be used in defining irreversi- 
ble reaction paths and mass transfer in nonequilibrium 
chemical systems by assuming that the overall reaction path 



Geologist, Twin Cities Research Center, U.S. Bureau ot Mines, Minneapolis, 
MN. 



can be represented by a series of partial equilibrium states be- 
tween product minerals and the aqueous phase as the system 
evolves toward a true equilibrium state. A reaction path model 
can predict component redistribution and variations in solu- 
tion composition that result from chemical reactions occurring 
in rock-water systems. By comparing the model's prediction 
with analytical data from laboratory experiments, it is possible 
to evaluate the validity of the reaction model and estimate its 
accuracy in describing less well-defined systems. 



50 



In recent reviews of the state of the art in geochemical 
modeling, Nordstrom (l-2) : and Jenne (3) compared the wide 
variety o\' computer programs currently available and de- 
scribed the strengths and weaknesses of individual programs in 
their mathematical construction and in their conceptual ap- 
proach. Simulation of in situ leaching requires a modeling pro- 
gram that can accept a wide variety of solution components, 
including leach target metals and related dissolution byprod- 
ucts that are not usually considered major constituents in 
natural waters. The program should solve rock-water interac- 
tions and simulate irreversible reaction progress in open 
systems as well as solve for the solution speciation of all com- 
ponents. Kinetic rate constraints derived from laboratory or 
field data should be easily incorporated into the algorithm for 
mineral dissolution or precipitation. In situ leach mining 
systems also require that the selected program be able to han- 
dle solutions that range in concentration from initially dilute 
conditions to highly concentrated conditions. Finally, the 
computer program should be usable on personal computers in 
mining offices where mainframe computers may not be 
available and should allow the user to easily adjust the input so 
that a number of in situ problems can be modeled. 

The computer program selected for initial Bureau efforts 
in in situ modeling research is the USGS program NEW- 
PHRQ, which is an updated version of computer program 
PHREEQE (4). NEWPHRQ is a FORTRAN computer pro- 
gram designed to model geochemical reactions based on an 
ion-pairing model. NEWPHRQ simulates many common 
hydrogeochemical phenomena such as mixing, rock-water in- 
teractions, mineral equilibrium, and ion-exchange reactions. 
The reaction path, system composition, temperature condi- 
tions, equilibrium conditions, aqueous species, and mineral 
phases are completely user definable, which presents a power- 
ful approach to modeling the unique conditions of in situ min- 
ing. NEWPHRQ is currently running on a Bureau personal 
computer. 



The PHREEQE family of geochemical models has been 
successfully applied to a number of hydrogeochemical prob- 
lems. Plummer (5) used reaction path calculations by PHREE- 
QE in conjunction with mass balance calculations to identify 
the effects that incongruent dissolution reactions have on 
ground water compositions within a carbonate aquifer. 
Marozas (<5) used techniques similar to Plummer's that com- 
bined PHREEQE's geochemical reaction path modeling with 
mass balance calculations to study the migration of base 
metals along ground water flow paths in desert alluvial basins. 

In order to extend the application of geochemical model- 
ing to in situ leach mining, several adjustments to the com- 
puter program are necessary. The current ion-pairing ap- 
proach used by PHREEQE and NEWPHRQ is based on the 
Debye-Hiickel mathematical expression for determining activi- 
ty coefficients of aqueous species appropriate for dilute 
aqueous solutions. However, Bureau experimental studies 
show that, when recycling lixiviants, in situ mining will gener- 
ate highly concentrated solutions, and thus it is important that 
the program be modified for solutions of high ionic strength. 
At higher concentrations, the chemical identity of the species 
begins to play an important role in the thermodynamics of the 
system. While some of this behavior can be described by em- 
pirical modifications to the Debye-Hiickel limiting law, such as 
the Davies equation, which is incorporated in NEWPHRQ, 
many important contributions are specific to the chemistry of 
the species. Recently developed phenomenological equations, 
known as the Pitzer equations, appear to have the required 
flexibility to describe specific ion interactions and have been 
successfully applied to a number of geochemical processes 
(7-8). Pitzer equations can be added to the NEWPHRQ code 
to accurately model highly concentrated leaching solutions and 
more closely characterize ore-body interactions with recycled 
lixiviants. 



POTENTIAL APPLICATIONS OF GEOCHEMICAL MODELING 



Geochemical models can characterize the chemical prop- 
erties of an in situ leach mining system in terms of its dominant 
variables — major and minor ions, oxidation-reduction poten- 
tial, acidity, complexing components, mineral stability, and 
adsorbing surfaces. Computer analyses of these data provide a 
systematic approach for defining the relative importance of 
different variables in determining metal recoveries expected 
during in situ mining. 

Computer models can calculate the equilibrium speciation 
of metals in solution if relevant stability constants are known. 
This approach presents a distinct advantage over laboratory 
methods, which are presently unable to determine unequivocal 
identification of solution species. The term "species" refers to 
the actual molecular form in which an element is present in 
solution. For example, copper may exist as one of the free ions 
Cu 1+ or Cu 2 + , or as a complex ion pair, such as Cu(OH) + , 
Cu(S0 4 ) , or CuCl + . Speciation can be one of the critical fac- 
tors in estimating metal recovery because the chemical 
behavior of any element depends on its chemical form in solu- 
tion. For example, adsorption of metals onto clay minerals 
and metal hydroxides can be either reduced or enhanced, 
depending on what metal complex exists in solution. At high 



-Italic numbers in parentheses refer to items in the list of references at the end 
of this paper. 



metal concentrations, the presence of complexes can affect the 
solubility of solids. Complex formation in solution among 
constituents derived from dissolving mineral lattices or with 
constituents of the lixiviant must be taken into account in ex- 
ploring mechanisms of metal transport by in situ leach mining 
solutions. 

Computer models can test the effects that the variation of 
system parameters will have on metal recoveries. Acidity ef- 
fects can be modeled by varying the pH input into the model 
and charting the consequent changes in metal concentration. 
Computer models can calculate the variations expected in ore 
mineral solubilities at different temperatures. Models can 
simulate temperature variations along flow paths to evaluate 
potential metal loss that is likely to occur during flow from 
near the bottom to the surface of recovery wells. Interference 
from gangue cations on metal recovery can be evaluated by 
changing input solution composition or increasing the amount 
of gangue reaction occurring along the reaction path. Kinetic 
controls on metal recovery can be tested by mathematically 
varying the dissolution rate of minerals into solution. Initially, 
the predictive capabilities of computer models can be verified 
by comparison with laboratory experiments and preliminary 
field tests; then the model can be extended to numerically 
simulate recoveries for conditions not duplicated in the 
laboratory or well controlled in the field. 



51 



RESULTS 



SPECIATION AND MINERAL STABILITY 
ANALYSIS 

Data from whole-core leaching experiments with sulfuric 
acid on oxide copper ores from deposits in Pinal County, AZ 
(9),'' were used to initiate research in modeling in situ leach 
chemistry. Analytical data from fluids collected at 48- to 72-h 
intervals were input into NEWPHRQ for each leach experi- 
ment. NEWPHRQ was then used to solve for component 
speciation distribution and for mineral stability during time 
steps throughout the experiment. The thermodynamic data 
base used for this analysis is from the compilation of Ball (10). 
Typical speciation and mineral stability results will be il- 
lustrated by detailed analysis of one of the Bureau's core- 
leaching experiments in which 1 -pet-copper chrysocolla ore 
from Santa Cruz was leached with 50 g/L sulfuric acid. Figure 
1 shows the pH variation versus the amount of solution re- 
covered in the experiment. 

Figure 2 shows how the relative concentration of copper 
species varies as a function of the amount of solution re- 
covered. The amount of solution recovered increases directly 
with time as sulfuric acid is added to the system. The data 
show that the speciation and therefore the chemical behavior 
of copper vary throughout the experiment. Initially, metal 
sulfate complexes dominate copper speciation in solution; 
however, as the acidity of the recovered solution increases and 
about 100 g of solution is recovered, free copper ion becomes 
the dominant species. This speciation distribution implies that 
there is less metal complexing and consequently relatively less 
metal mobility in solution as acidity increases. The causes 
behind the loss of sulfate complexing can be investigated by 
analysis of the speciation behavior of sulfate in the system. 

Figure 3 is a plot of sulfate speciation in the experiment and 
shows the relative amount of sulfate that is present as metal 
sulfate complexes, free sulfate ions, and bisulfate ions. Ini- 
tially, free sulfate anions and metal sulfate complexes domi- 
nate the sulfate system, but with time, acidity increases and 
sulfate reacts to bisulfate as reaction A is driven to the right. 



H + +S0 4 =-HS0 4 



(A) 



The fraction of hydrogen sulfate increases at the expense of 
metal sulfate complexes, and consequently, metal mobility 
may be reduced. Speciation analyses of particular systems can 
help determine the lowest acidity required for continued cop- 
per oxide dissolution yet maximize metal mobility in solution. 
Further information on copper behavior in solution is de- 
rived from calculations of the activity or the effective concen- 
tration of a species in an electrolyte solution. Evaluation of 
metal activity in solution is important because it is from this 
measurement that the solubilities of minerals in the leaching 
environment can be calculated. The activity of solution com- 
ponents is related to measured molalities by the equation 



where 
and 



m S 7s = a s> 

m s = molality of species S, 

7 S = activity coefficient of species S, 

a s = activity of species S. 



(1) 



Activity coefficients are determined for each species in 
NEWPHRQ by the user's choice of either an extended Debye- 
Hiickel formula or the Davies formula. Both of these formulas 



'The deposits are owned by Cyprus Casa Grande Corp. and by ASARCO San- 
ta Cruz Inc. and Freeport Copper Co. 



calculate the change in activity coefficients as a function of 
ionic strength. 

There is a considerable difference between measured con- 
centrations of total copper and the calculated effective copper 



x 
a 




200 300 400 500 
SOLUTION RECOVERED, g 



600 



Figure 1.— The pH of leach solutions from 50 g/L whole-core 
leach experiments on 1.0-pct-copper chrysocolla ore from 
Santa Cruz. 



| 0.8 
o 



o .6 
E 

</) 

LU 

O .4 

Ld 

Q. 

3 
O o 




OCu + */total Cu 
a Cu(S0 4 )/total Cu 



100 



200 



300 



400 



500 



600 



SOLUTION RECOVERED, g 



Figure 2.— Mole fraction of copper in experimental leach 
solutions present as free metal ion and copper sulfate 
complex. 




DSO«/total S 

HSOVtotal S 

O Metal(S0 4 )/total S 



100 200 300 400 500 
SOLUTION RECOVERED, g 



600 



Figure 3.— Mole fraction of sulfate in experimental leach 
solutions present as sulfate ion, bisulfate, or complexed with 
cations. 



52 



concentration or activity in the leaching solutions from this ex- 
periment. Figure 4 shows the total concentration of aqueous 
copper regardless of its form, which includes free or hydrated 
copper ion and copper sulfate complexes. The molality and the 
activity of free copper ion are also shown. The data show that 
there is a wide variation in total copper concentration, relative 
to the variation in the activity of free copper. A simple model 
of copper fluctuation, therefore, may apply to in situ systems 
that need not account for wide variations in measured copper 
concentration but rather can focus on explaining the less 
variable activity of copper. Future research goals of the 
Bureau are to use computer modeling analyses of the variation 
in copper activity to provide insights into the kinetic and 
equilibrium mechanisms that control the in situ leaching of 
copper. 

The activity of copper is emphasized here because it is used 
for determining the saturation state of mineral phases in the 
system. The saturation state of an aqueous system, with 
respect to mineral phases, can be calculated by dividing the ion 
activity product (IAP) by the solubility product (K sp ) constant. 
The IAP is the product of the activities of the reaction prod- 
ucts, each raised to the power indicated by its numerical co- 
efficient, divided by the product of the activities of the re- 
actants raised to the power of their numerical coefficients. The 
log of the IAP/K sp ratio is called the saturation index (SI). If a 
solution is supersaturated with respect to a particular mineral, 
the SI is positive and the mineral has a tendency to precipitate. 
If the solution is undersaturated, the SI is negative and the 
mineral has a tendency to dissolve. If a mineral is at equi- 
librium, then the SI is zero. 

Figure 5 is a plot of SI versus solution recovered for three 
mineral phases in the Santa Cruz experiment. The SI line for 
amorphous silica is close to zero or equilibrium throughout the 
experiment. This implies that the leaching system responds to 
the addition of silica from the dissolution of chrysocolla or 
clays by precipitation of amorphous silica. The gypsum SI line 
is close to equilibrium early in the experiment but falls to 
undersaturated levels later. This implies that gypsum precipi- 
tation would be a problem only in the early stages of in situ 
leaching. The chrysocolla SI line indicates that it is at first 
supersaturated but later becomes undersaturated in the experi- 
ment. The calculated supersaturation of chrysocolla may have 
several implications. First, there may be a kinetic inhibition to 
precipitation during the supersaturation interval, which keeps 
copper in solution long enough for recovery in the experiment. 
However, this may not be the case in the field, where time to 
overcome activation energy barriers to precipitation may be 
within the residence time of the lixiviants in the host ore. Sec- 
ond, because the activity of copper remains near steady state, 
this may indicate that there is another, faster control affecting 
copper concentration in solution other than chrysocolla 
dissolution, such as the presence of unidentified metastable 
phases. Finally, the chrysocolla solubility data used in the 
calculation may need to be revised to account for the solid 
solution identified by microprobe studies of chrysocolla in the 
drill core. 

APPLICATION MODELS 

The most powerful application of computer modeling 
programs, such as NEWPHRQ, to in situ mining is the pro- 
gram's ability to follow user-defined hypothetical reaction 
paths and to calculate the resulting solution compositions. 
Given a set of realistic variables, a reaction path model can 
predict expected metal recoveries, gangue mineral in- 
terferences, and lixiviant losses. Conversely, given unexpected 



metal recoveries, reaction path models can test variables that 
might be causing anomalies and may suggest a means to max- 
imize positive variances and minimize negative variances. 

The following sections present several reaction path 
models that simulate the effect of several variations in system 
parameters on the leaching of Santa Cruz 1.0-pct-copper 
chrysocolla ore with 50 g/L sulfuric acid. These models are 
presented to demonstrate potential applications of computer 
modeling for in situ mining. 

The copper recoveries specified in figures 6 to 13 are cal- 
culated for certain theoretical conditions and therefore should 
not be taken as the copper recovery expected for an actual field 
situation. Rather, the intent of this study is to show how ac- 
curate computer models can be integrated into systematic 
methods of predicting copper recovery under a given set of 
conditions. 

Effects of Sulfate Complexing on Copper Recovery 

Cations in an aqueous medium react with available bases 
to improve the stability of electrons in their outer shell. Metal 
ions, such as copper, can react to achieve such stabilization 
without the formation of precipitates by forming metal coordi- 
nation complexes with anions or negatively charged molecules 
called ligands. It is well known that the presence of foreign 




200 300 400 500 

SOLUTION RECOVERED, g 



600 



Figure 4.— Concentration of total copper and molality and 
activity of free copper ion in experimental leach solutions. 



a. 

< 



x 

Ul 

a 



O Amorphous SI0 2 
a Gypsum 

Chrysocolla 



O - 



< 
cc 

=> 



< 

V) 




600 



SOLUTION RECOVERED, g 



Figure 5.— SI of mineral phases with respect to experi- 
mental leach solutions. Zero line represents equilibrium; 
positive values indicate supersaturation, and negative values, 
undersaturation. 



53 



ligands increases the solubility of slightly soluble metal salts 
(11). Sulfate in sulfuric acid lixiviants increases the solubility 
of chrysocolla during in situ mining. 

It is possible to illustrate by computer modeling the sig- 
nificance of complex formation in enhancing the solubility of 
copper solids. In this model, chrysocolla dissolves to equi- 
librium with solutions of varying acidity, while equilibrium is 
also maintained with amorphous silica — first, with no sulfate 
present and, second, with excess sulfate present for complex- 
ing. Chrysocolla solubility was estimated from 50-g/L sulfuric 
acid leach experiments. 

Results show (fig. 6) that copper mobility is significantly 
increased by sulfate complexation in solution. Copper re- 
coveries are approximately 1 l A times higher when sulfate is 
present at a pH of about 0.8. 

Effects of Gangue Cations on Copper Recovery 

Gangue cations compete with copper for sulfate ligand in 
solution, and because alkali-earth sulfate aqueous complexes 
are more stable than copper sulfate complexes, sulfate ligand 
will preferentially complex with gangue cations, leaving copper 
in solution as free copper ion. The presence of sulfate-seeking 
cations from the dissolution of gangue minerals, in addition to 
copper, has the effect of forcing copper to its free ion state. If 
free copper is abundant in solution, copper minerals such as 
cuprite, chalcanthite, or native copper may approach satura- 
tion levels and precipitate. High concentrations of gangue cat- 
ions may hinder the transport of copper by forcing precipita- 
tion, even at low pH values. 

The effects of gangue cations at low pH on copper re- 
coveries were simulated by using NEWPHRQ to calculate the 
total amount of copper that would enter solution during the 
equilibrium dissolution of chrysocolla in the presence of 
gangue cations at various pH levels. Theoretical copper re- 
coveries were calculated for a system with (1) no gangue cat- 
ions in solution, (2) gangue cations present at concentration 
levels typical of those found in the late stages of the Santa 
Cruz experiment (178 ppm calcium, 396 ppm magnesium, 
1,090 ppm iron, and 1,390 ppm aluminum) and with 49,000 
ppm sulfate, and (3) twice as many gangue cations in solution 
as those in the Santa Cruz experiments (356 ppm calcium, 792 
ppm magnesium, 2,180 ppm iron, and 2,780 ppm aluminum) 
and 24,500 ppm sulfate. The last model might simulate condi- 
tions expected during the recycling of in situ leach solutions. 

The results of these simulations (fig. 7) show that gangue 
cations interfere with copper mobility in solutions during the 
equilibrium dissolution of chrysocolla in sulfuric acid lixiviant. 
This model can predict at what level gangue mineral inter- 
ferences with copper mobility will present an economically 
unacceptable situation. 



Effects of Ionic Strength on Copper Recovery 

An efficient way to lower mining costs during in situ min- 
ing is to reuse lixiviants that maintain sufficient acidity to 
dissolve copper oxide minerals. During this recycling, the con- 
tinuous dissolution of ore and host minerals increases the con- 
centration of solutes in solution and thereby raises the ionic 
strength of recycled solutions. The effective concentration of 
metals in electrolyte solutions, and therefore the solubilities of 
ore minerals, are dependent on ionic strength. As ionic 
strength increases, solute molecules will, on the average, be 
closer together and interactions in solution will take place 
more readily. 



Because the activity of copper changes with ionic 
strength, solubilities will vary with ionic strength. NEWPHRQ 
was used to investigate the effects of increasing ionic strength 
on total copper recovery. In order to observe only the effects 



0.7 



.6 



o 
E 



Ld 

> 

o 
o 

Ld 

or 
o 



KEY 
S0 4 complexing 

OWith 
□ Without 



0.6 




1.8 



pH 



Figure 6.— Equilibrium concentration of copper in presence 
and absence of sulfate in solutions saturated with amorphous 
silica. 




pH 

Figure 7.— Equilibrium concentration of copper in solutions 
saturated with amorphous silica. A, No gangue cations; 6, 
gangue cation concentrations are typical of one-pass leach 
experiments with 0.54 mol/L sulfate; C, gangue cation concen- 
trations are twice that of one-pass leach experiments with 0.26 
mol/L sulfate. 



54 



of ionic strength variations without gangue cations or ligand 
concentration effects as previously considered, ionic strength 
in the model was changed by the addition of potassium and 
chlorine, which do not interfere with copper or copper com- 
plexes in solution. Potassium and chlorine were added in con- 
centrations sufficient to reach the desired level of ionic 
strength. 

The results (fig. 8) show that, as ionic strength increases at 
the same pH, copper recovery is decreased. As ionic strength 
increases, the activity coefficient for copper increases to values 
greater than one, so that the effective concentration of copper 
increases. This inhibits the solubility of chrysocolla because 
saturation is achieved at lower copper concentrations. This 
model uses the Davies equation for the calculation of activity 
coefficients, but as was discussed in an earlier section, a 
specific interaction model would probably describe the solu- 
tion behavior more accurately at high ionic strengths. 

Effects of Increasing Gangue Cations 
on Clay Mineral Precipitation 

Metal cations in solution are known to adsorb onto clay 
minerals in rock-water systems. Solutions concentrated with 
gangue cations can approach clay mineral saturation levels 
even at low pH. When clay minerals precipitate in in situ 
environments, surface adsorption sites are provided for the ad- 
sorption and consequent loss of metal from solution. 

NEWPHRQ was used to model the saturation state of 
clay minerals during the concentration of gangue cations. 
Silica concentrations in solution were held at equilibrium with 
amorphous silica, and copper concentrations were held con- 
stant at 5,000 ppm. Gangue cation concentrations were gradu- 
ally increased at a system pH of 1 .0 and at a system pH of 3.0. 

The saturation states of potassium-illite and of mag- 
nesium-beidellite minerals in these hypothetical solutions is il- 
lustrated by plotting the SI of each mineral (fig. 9). The zero 
line represents equilibrium; when the SI plots above this line, 
clay minerals precipitate, while at values below this line, clay 
minerals dissolve. The results show that potassium clay 
minerals approach saturation as potassium in solution reaches 
0.05 mol/L, regardless of pH. Magnesium clays do not ap- 
proach equilibrium even at magnesium concentrations of 1 
mol/L. Models can predict when gangue cation buildup in re- 
cycled solutions might begin precipitating clay minerals. 

Effects of Potassium Exchange Reactions 
on Copper Recovery 

Bureau laboratory experiments indicate that a con- 
siderable amount of acid is consumed by ion-exchange reac- 
tions in the early phases of whole-core leaching. This acid is no 
longer available to react with copper oxide minerals, which 
decreases the efficiency of in situ leaching. Reduction of this 
initial acid consumption may be required to economically 
leach copper. 

NEWPHRQ was used to model the effect of adding 
potassium to the lixiviant while maintaining a constant number 
of exchange sites, chrysocolla equilibrium, and amorphous 



0.7 



o 


.5 


E 




„ 




o 


.4 


Ul 




QL 




Ul 




> 


..5 


o 




o 




Ul 




az. 


.2 


3 




o 





.1 - 



KEY 


Ionic strength 


O0.8 

□ 1.0 
4.3 



0.6 




1.8 



pH 



Figure 8.— Equilibrium concentration of copper in solutions 
of varying ionic strengths saturated with amorphous silica. 
Zero line represents equilibrium. 




K-illite 
Mg-beidellite 

_J l 



-5 -4 -3 -2 -1 
LOG (PREDOMINANT CATION), mol 



Figure 9.— SI of clay minerals in 50 g/L sulfuric acid, amor- 
phous silica-saturated solution with varying concentrations of 
gangue cations. 



55 



silica equilibrium. The pH of the system is adjusted as ex- 
change and dissolution reactions occur. Potassium affects the 
reaction by limiting the amount of cations exchanging for 
hydrogen ion at clay exchange sites. Consequently, acid loss 
from exchange reactions is diminished and chrysocolla dissolu- 
tion is enhanced. 

Figure 10 is a plot of copper recovered from the leaching 
of chrysocolla ore as a function of potassium concentration in 
the lixiviant. Copper recovered without added potassium is 
shown by the "potassium-free" line. This figure shows that 
significant differences in copper recoveries are expected only 
when potassium exceeds 0.01 mol/L in the solution. The eco- 
nomic feasibility of adding potassium or other outside reagents 
to lixiviants for copper recovery enhancement can be deter- 
mined by computer estimations of how much of the reagent 
will be required to significantly increase copper recoveries. 

Effects of Biotite Dissolution on Copper Recovery 

Characterization and leaching studies on Santa Cruz ore 
(9) have shown that the presence of biotite in the rock matrix 
inhibits the leaching of copper. Biotite dissolves in acid, which 
removes acid from the lixiviant, increases the pH, and pro- 
motes copper mineral precipitation. Gangue cations released 
by biotite dissolution compete with copper for sulfate ligand 
and consequently can inhibit copper mobility in solution. 

Biotite interference in copper leaching can be simulated 
with computer models. The model simulates the dissolution of 
biotite into a system in which equilibrium is maintained with 
chrysocolla and amorphous silica at low pH. The amount of 
biotite that dissolves is increased throughout the model to 
gauge the effects of increased biotite reactivity in the system. 

Figure 11 shows that copper recovery decreases by an 
order of magnitude as 0. 15 mol of biotite dissolve. The extent 
of biotite mineralization in potential ore bodies can be esti- 
mated by petrologic examination and the consequent effects of 
biotite reactivity on copper mobility projected by computer 
modeling. 

Effects of Copper Dissolution Rates 
on Copper Recovery 

Characterization work on thin sections of oxide ore 
deposits (9) has revealed that copper is present in two different 
mineral phases — chrysocolla and copper-bearing clays. 
Chrysocolla is principally fracture hosted, while copper- 
bearing clays are principally disseminated in the rock matrix. 
The amount of copper released by solubilization of copper ore 
depends on the dissolution kinetics of the two different phases. 
Laboratory leaching studies on this type of ore show that cop- 
per recovery reaches a maximum in the early stages of leaching 
(fig. 12) and then decreases to a near-constant level for the re- 
mainder of the experiment. 

Path-dependent reaction progress models are simulated 
by NEWPHRQ as a series of steps where the resulting solution 
from one step becomes the starting solution for the next step 
of the model. NEWPHRQ can simulate expected copper re- 
covery from two different mineral phases by varying the rate at 




-6 -4-2 2 

LOG (K ADDED TO SOLUTION), mol/L 

Figure 10.— Equilibrium concentration of copper in 50 g/L 
sulfuric acid, amorphous silica-saturated solution as function 
of potassium concentration in lixiviant. "Potassium-free" line 
marks copper concentration level when no potassium is pres- 
ent. 



0.24 



o 



o 

UJ 

on 

Ld 
> 

o 
o 

UJ 
Q£ 

3 
O 




0.20 



BIOTITE DISSOLVED INTO SOLUTION, mol 

Figure 11.— Equilibrium concentration of copper in 50 g/L 
sulfuric acid, amorphous silica-saturated solution as function 
of increasing biotite reactivity. 



56 



which each phase is added to the system, allowing the solution 
to come to equilibrium with amorphous silica and passing the 
solution on to the next step. Depletion of fracture-hosted 
chrysocolla is simulated by stopping the chrysocolla dissolu- 
tion reaction while maintaining the matrix clay dissolution at 
the same rate as before. Acid is added at each step to simulate 
the continual influx of acid to the system. 

The results of the model are shown in figure 12, where a 
peak similar to that observed in the laboratory experiment is 
generated. The results of this model suggest that differing 
dissolution kinetics of copper-bearing phases are potential 
controls on copper recoveries in the experiment. This example 
illustrates one way that computer simulations may explain the 
reactions responsible for copper release and subsequently can 
predict expected copper recoveries. 

Effects of Temperature Variations 
on Copper Recovery 

Temperature variation during in situ mining may affect 
copper recoveries. Temperatures may vary considerably when 
ore deposits are leached at depth in areas with high thermal 
gradients. The basic relationships that describe the influence 



0.6 



1 1 1 

Chrysocolla dissolution ceases 



O Model 

D Experiment 




8 



pH 



Figure 12.— Effects of two different rates of ore mineral 
dissolution on copper concentration. Experimental data are 
from 50 g/L sulfuric acid leach experiments on 1-pct-copper 
chrysocolla ore. At the point indicated on the diagram, 
chrysocolla dissolution ceases while copper-clay minerals 
continue to dissolve. 



of temperature on the equilibrium constant of a chemical reac- 
tion or phase equilibrium require knowledge of the enthalpy of 
reaction, the heat capacity of the reaction, and the variation of 
the heat capacity with temperature. Heat capacity data on 
chrysocolla solid solutions are limited or unavailable, and en- 
thalpy data must be estimated. Until more accurate data are 
obtained from direct calorimetry and by measurement of equi- 
librium properties of chemical reactions, predictions of copper 
recovery by chemical models are necessarily hypothetical. The 
following is presented only as an example of how models can 
calculate copper recoveries when thermodynamic values for 
the mineral phases of interest in the system are assessed. 

Computer simulations are especially useful for modeling 
changing temperature conditions because of the complexity of 
the effects of temperature on the entire system. In general, in- 
creasing temperature favors dehydration reactions, solubiliza- 
tion of chrysocolla, and ion association reactions, while acid 
dissociation diminishes. 

The model used to demonstrate temperature variation ef- 
fects on in situ mining was designed so that chrysocolla dis- 
solves to equilibrium as the pH varies and the system is main- 
tained in equilibrium with amorphous silica. Figure 13 il- 
lustrates how temperature can affect copper recovery when 
calculated with an estimated chrysocolla enthalpy. Results 
show that at similar acidity levels more copper is recovered at 
100° C than at 25° C. 




pH 

Figure 13.— Equilibrium concentration of copper in 50 g/L 
sulfuric acid, amorphous silica-saturated lixiviant at 25° C and 
100° C. 



57 



SUMMARY 



It has been demonstrated that computer modeling pro- 
grams have applications for in situ mining through the quanti- 
fication of factors that affect the leaching chemistry. To esti- 
mate ore recovery and lixiviant losses, knowledge of ore 
mineral solubility and in situ fluid-rock interactions is essen- 
tial. Geochemical models can characterize the chemistry of in 
situ systems in terms of the speciation of solution components 
and can identify those mineral phases that should dissolve and 
those that should precipitate under leach mining conditions. 

Several models have been developed to demonstrate how 
computer simulations can be used to predict ore recoveries for 
a variety of conditions not evaluated by laboratory testing. 
These hypothetical models calculate the effects of different 
copper dissolution rates on the behavior of copper recovery 
curves, the effects of preflushing with potassium to diminish 
hydrogen exchange, and the effects of temperature on chryso- 
colla solubility. 



Although it is generally known that highly reactive gangue 
constituents in the host matrix can consume acid and raise 
mining costs, mathematical treatment of gangue reactivity can 
project acid consumption and establish at what gangue con- 
centrations it would no longer be economically feasible to 
leach-mine. Several different models that address the impact 
of secondary reactions on copper recovery show the wide 
variety of approaches to this problem that computer simula- 
tions make possible. 

The models presented in this paper are initial attempts to 
use computer simulations for predicting copper behavior dur- 
ing in situ leaching. Future work by the Bureau will focus on 
continued refinement of the geochemical models by calibra- 
tion of the data base with experimental results, by the incor- 
poration of known kinetic constraints into the program, and 
by testing of the program under actual field conditions. 



REFERENCES 



1. Nordstrom, D. K., L. N. Plummer, T. M. L. Wigley, T. J. 
Worley, J. W. Ball, E. A. Jenne, R. L. Bassett, D. A. Crerar, T. ML 
Florence, B. Frit, M. Hoffman, G. R. Holdren, Jr., G. M. Lafan, S. 
V. Mattigod, R. E. McDuff, F. Morel, M. M. Reddy, G. Sposito, and 
J. Thailkill. A Comparison of Computerized Chemical Models for 
Equilibrium Calculations in Aqueous Systems. Ch. in Chemical 
Modeling in Aqueous Systems, ed. by E. A. Jenne. ACS Symp. Ser. 
93, 1979, pp. 857-892. 

2. Nordstrom, D. K., and J. W. Ball. Chemical Models, Computer 
Programs and Metal Complexation in Natural Waters. Ch. in Com- 
plexation of Trace Metals in Natural Waters, ed. by C. J. M. Kramer 
and J. C. Duinker. Nijhoff (The Hague)-Junk Publ., 1984, pp. 
149-164. 

3. Jenne, E. A. Chemical Modeling — Goals, Problems, Ap- 
proaches and Priorities. Ch. in Chemical Modeling in Aqueous 
Systems, ed. by E. A. Jenne. ACS Symp. Ser. 93, 1979, pp. 3-21. 

4. Parkhurst, D. L., D. C. Thorstenson, and L. N. Plummer. 
PHREEQE — A Computer Program for Geochemical Calculations. 
U.S. Geol. Surv. WRI 82-14, 1982, 29 pp. 

5. Plummer, L. N., D. L. Parkhurst, and D. C. Thorstenson. 
Development of Reaction Models for Groundwater Systems. Geo- 
chim. et Cosmochim. Acta, v. 47, 1983, pp. 665-685. 



6. Marozas, D. C. The Effects of Mineral Reactions on Trace 
Metal Characteristics of Groundwater in Desert Basins of Southern 
California. Ph.D. Thesis, Univ. AZ, Tucson, AZ, 1987, 116 pp. 

7. Harvie, C. E., and J. H. Weare. The Prediction of Mineral 
Solubilities in Natural Waters: The Na-K-Mg-Ca-Cl-S0 4 -H 2 System 
From Zero to High Concentration at 25° C. Geochim. et Cosmochim. 
Acta, v. 44, 1980, pp. 981-997. 

8. Harvie, C. E., N. Moller, and J. H. Weare. The Prediction of 
Mineral Solubilities in Natural Waters: Na-K-Mg-Ca-Cl-S0 4 -OH- 
CO3-CO2-H2O System to High Ionic Strengths at 25° C. Geochim. et 
Cosmochim. Acta, v. 48, 1984, pp. 723-751. 

9. Cook, S. S., and S. E. Paulson. Leaching Characteristics of 
Selected Supergene Copper Ores. Soc. Min. Eng. AIME preprint 
88-195, 1988, 15 pp. 

10. Ball, J. W., D. K. Nordstrom, and E. A. Jenne. Additional and 
Revised Thermochemical Data and Computer Code for WATEQ2 — A 
Computerized Chemical Model for Trace and Major Element Specia- 
tion and Mineral Equilibria of Natural Waters. U.S. Geol. Surv. WRI 
78-116, 1980, 109 pp. 

11. Stumm, S., and J. J. Morgan. Aquatic Chemistry. Wiley, 1981, 
780 pp. 



58 



MODELING INFILTRATION TO UNDERGOUND MINE WORKINGS 
DURING BLOCK-CAVE LEACHING 

By Robert D. Schmidt 1 



ABSTRACT 

The U.S. Bureau of Mines has modeled the infiltration of leach solution to an 
underground mine void during block-cave copper leaching. The underground leachant col- 
lection area is represented as a distributed hydrologic sink, referred to as an "area sink." The 
area sink solution, which was developed previously using the analytic element method, is 
described briefly. The area sink model is calibrated against field data from well "drop" tests 
and an in-mine flow survey; it is then used to estimate the size and shape of the free saturated 
plume of leachant that exists above the mine void during leaching. An inverse problem is 
posed, using area sink and well solutions to estimate the leakage coefficient and permeability 
of ore material above the mine during ongoing leaching operations. 



INTRODUCTION 



Understanding and controlling the distribution of leach 
solutions in unsaturated ore material is essential for con- 
ducting safe and efficient leaching operations above the am- 
bient water table. The development of a distributed area sink 
solution for modeling infiltration to underground mine work- 
ings during block-cave leaching is part of a Bureau of Mines 
effort to provide the industry with modeling tools that 
facilitate understanding of in situ leaching hydrology in both 
saturated and unsaturated flow settings. 



A distributed area sink solution, developed using the 
method of analytic elements and previously used to model 
localized aquifer infiltration from surface sources such as 
ponds, rivers, and lakes, has been adapted for modeling in- 
filtration to irregularly shaped underground mine workings. 2 
The solution, which is highly efficient, has been implemented 
in a Bureau hydrology computer program, MINEFLO. This 
paper describes the application of the area sink solution to the 
hydrologic setting of a block-cave leaching operation near San 
Manuel, AZ. 



METHOD OF BLOCK-CAVE COPPER LEACHING 



Block-cave copper leaching is a method by which copper 
is leached in situ from low-grade ore that has been at least par- 
tially rubbled by prior conventional block-cave mining ac- 
tivities. In one variation, shallow wells are constructed on the 
surface of a rubbled oxide ore mass and an acidic leach solu- 



tion is injected into the wells. The leach solution flows 
downward through the rubbled mass until it reaches an ex- 
isting underground mine drift. The mineral-bearing solution is 
then collected in the mine workings, pumped to the surface, 
and processed to extract metal values. 



'Hydrologist, Twin Cities Research Center, U.S. Bureau of Mines, Min- 
neapolis, MN. 



2 Strack, O. D. L. Groundwater Mechanics. Prentice-Hall, 1987, 643 pp. 



59 



Figure 1 is a schematic cross section showing the setting of 
a block-cave copper-leaching operation near San Manuel, AZ. 
The entire setting is situated above the local ambient water 
table. The rubbled zone in figure 1 is low-grade oxide ore that 
was left behind following conventional block-cave mining of 
an underlying sulfide ore. The thickness of the rubbled ore 
mass in figure 1 averages 1,626 ft. Injection wells are drilled 
from the surface to an average depth of 910 ft. The bottom 200 
ft of each well is slotted pipe. The underground mine workings 
in figure 1 consist of an undercut drift at elevation 1,125 ft 
mean sea level (msl) and a haulage drift 65 ft below it, at eleva- 
tion 1,060 ft msl. The haulage level and the undercut level are 



connected by vertical transfer raises. The underground work- 
ings that serve as a collection drift for injected leach solution 
are on the haulage level. 

The cascade area in figure 1 is a zone of partially rubbled 
ore, which does not directly overlie the undercut workings. 
Ore material in the cascade area is characterized by mining- 
induced fractures or crack lines, which run parallel to the 
boundary of the undercut workings and dip at an angle of 
about 50°. There is no evidence that these fractures intercept 
the haulage workings. Thus, there are almost no permeable 
connections between the cascade area and the underlying 
mine. 




Figure 1.— Schematic cross section of block-cave leaching. 



60 



AVAILABLE HYDROLOGIC FIELD DATA 



At the time of this study, block-cave leaching operations 
had been ongoing at this site for about 72 weeks. Field data 
that describe the hydrologic setting during a 16-week interval 
of the mining operation, from weeks 26 to 41, consist of the 
following: (1) a daily history of the total solution injection rate 
in wells and the total solution recovery rate inside the mine, (2) 
an underground survey of mine inflow conducted during week 
41, and (3) the results of steady-state "drop" tests conducted in 
three wells during week 26. These data are used to calibrate a 
model of saturated conditions at the site. 



INJECTION AND RECOVERY HISTORY 

The total daily rates of solution injection and recovery 
during the first 72 weeks of leaching operations are plotted in 
figure 2. Variation in total injection rate during this period was 
due mainly to variation in the number of operating wells. The 
operational life span of injection wells averaged about 24 
weeks. Well losses occur as a result of subsidence and plug- 
ging. 

Fourteen wells were in operation in the interval from 
weeks 26 to 41. As figure 2 indicates, the injection and 
recovery rates during this period were relatively stable; injec- 
tion averaged 0.85 million gal/d, and recovery averaged 0.4 
million gal/d. During this period, about 0.45 million gal/d of 
injected solution was going into phreatic (free-surface) 
storage; thus, the saturated plume was expanding. 



WELL DROP TESTS 

Also shown in figure 3 are the locations of three wells in 
which drop tests and permeability slug tests were conducted. 
During these tests, wells 38, 42, and 45 were shut down one at 
a time, so that 13 injection wells remained in operation. 3 The 
decline in solution level was monitored in each well for about 
30 h. Steady-state levels were reached in all three wells within 
about 2 h after the start of the tests. Table 1 gives the steady- 
state solution level and a local permeability estimate (from slug 
tests) for each well. The steady-state elevations in table 1 are 
evidence that a stable piezometric surface exists from 150 to 
650 ft below the rubbled surface, in the immediate vicinity of 
operating injection wells. 



'Erskine, C. Hydrologic Drop Test In Situ Leach Operations. Magma Mining 
Co. internal rep., Mar. 1987, 5 pp.; for inf., contact C. Erskine, Pinto Valley 
Mining Co., Miami, AZ. 



Table 1.— Drop test results in three wells 

Time to reach 

steady state, 

min 

110 

50 

130 

'Above the undercut level. 

2 From slug test conducted in a nearby well. 





Well 


Time to reach 

steady state, 

min 


Steady-state 

elevation 

ft msl 


Effective 

permeability 

ft/s 


Saturated 

thickness, 1 

ft 


38 
42 

45 




110 

50 

130 


2,590 
2,153 
2,575 


2 6 x 10" 7 

6 x 10~ 5 

7 x 10- 6 


1,465 
1,028 
1,450 



MINE INFILTRATION SURVEY 

An in-mine survey to locate the transfer raises where leach 
solution was either flowing or dripping into the mine was con- 
ducted on the haulage level during week 41. Figure 3 is a plan 
view of the mine workings, showing the results of this survey. 
The outlines of the undercut workings and the haulage work- 
ings are shown in this figure in relation to the 14 overlying in- 
jection wells. The locations of 94 flowing and dripping transfer 
raises are represented on this figure as black rectangles. Dry 
transfer raises are not marked. The surveyed area of inflow in 
figure 3 is called the in-mine collection area. Because of the 
low permeability of undisturbed ore, there was no measurable 
infiltration to haulage workings located beneath the cascade 
area, northwest of the undercut drift in figure 3. 

The steady infiltration of leach solution observed in the 94 
transfer raises indicates that saturated conditions had 
developed in a substantial portion of the overlying rubbled 
material owing to 41 weeks of continuous solution injection. 
Further, the locations of flowing and dripping transfer raises 
south and east of the injection wells in figure 3 suggest the ap- 
proximate location of the saturated boundary as it exists above 
the undercut workings, during week 41. 



KEY 

Injection 
Recovery 




i i i i i i i i 

10 20 30 40 50 80 70 80 

WEEK 

Figure 2.— Injection and recovery history through week 72. 



61 



800 



600- 



LU 

U 

< 

r- 

— 
a 



400- 




200- 



LEGEND 

Injection well collar 

h Dripping transfer raise 

1 Flowing transfer raise 

ii Haulage drift 
II (elev 1,064 ft) 

I — | Undercut drift 
1 — ' (elev 1,124 ft) 



t r 

200 400 600 

DISTANCE, ft 



800 



Figure 3.— Plan-view injection well locations and collection area. 



MODELING MINE INFILTRATION USING DISTRIBUTED AREA SINKS 



In two-dimensional flow problems, an area sink is a 
distribution of sinks over an area. Steady-state infiltration 
from a saturated aquifer into an overlying or underlying 
underground mine void can be modeled, in plan view, by one 
or more distributed area sinks, provided the size and shape of 
the area sinks can be chosen arbitrarily, to conform to the 
distribution of infiltration into the mine. In such a model, the 
rate of infiltration per unit area of mine workings, y, is given 
by 

y=*^®-, (1) 

where 0* - </>(z) is the difference in piezometric elevation be- 
tween the mine void and the overlying (or underlying) aquifer 
at a location z; z is a complex variable used to represent a two- 
dimensional plan view coordinate location; and c is the 
resistance to vertical flow through a layer of saturated material 
located above (or below) the mine. By application of Darcy's 
law in equation 1, 



c = -j- 



(2) 



where h is the thickness of the resistance layer and k is its ver- 
tical permeability. The use of a resistance parameter in equa- 
tion 1 to describe both the permeability and the thickness of a 
saturated layer implies that flow through the resistance layer is 
assumed to be one-dimensional. 

A distributed area sink solution that satisfies equation 1 at 
every point z in an aquifer above (or below) a mine void can be 
obtained by integrating the solution for a point sink over an 
area. This integration is cumbersome, however, and may be 
carried out analytically for only a few, relatively simple 
shapes. Alternatively, a solution developed using the method 
of analytic elements can be used to approximate the distribu- 
tion of piezometric head, <j>(z), at every point z in the aquifer. 
The advantage of the analytic element methods is in the ease 
with which an approximate solution can be obtained for flow 
problems involving arbitrarily configured mine voids. 



62 



DISTRIBUTED AREA SINK SOLUTION USING ANALYTIC ELEMENTS 



A full derivation of the distributed area sink solution 
using the method of analytic elements, together with a discus- 
sion of its use in representing infiltration sources such as 
ponds, rivers, and lakes, is given by Strack. 4 For com- 
pleteness, a brief outline of the method is presented here, with 
particular reference to conditions of underground mine in- 
filtration. 

Making use of the Dupuit-Forchheimer assumption, an 
area sink potential, <£ = <t>(0,h,k), can be defined as a func- 
tion of piezometric head, <$> = </>(z), saturated thickness, h, 
and permeability, k, that is continuous in both confined and 
unconfined mine inflow settings. 

Considering a contour C enclosing a domain D + to be 
the arbitrary plan-view boundary of an underground mine 
void and using 7 to denote a constant distribution of inflow to 
(or outflow from) the mine, then following Strack, an area 
sink solution for representing the mine void consists of deter- 
mining an area sink potential, <i> = $(z), at a location z, which 
fulfills the differential equation 



V 2 <J> 

for any z in D + , 

and the differential equation 



= 7 



V 2 <I> = 



(3) 



(4) 



for any z in D", 

where D" is outside of C. 

The problem is solved by splitting the potential <I> into two 
parts: 



d> = <t>i + <i>% 



(5) 



where 4>' is any function that satisfies V 2 <i>' = 7 for z in D*, and 
where <£' = in D". By itself, <£' is discontinuous across the 
boundary C and therefore violates the conditions for continui- 
ty of both pressure and flow; 4> e is used to eliminate these 
discontinuities. In order for the potential <J> (and therefore the 
pressure head 0) to be continuous on the boundary, <i> e must 
jump by an amount 

<|>e-_<i>e += _$i ( 6 ) 

across the boundary, where the superscript ( + ) and ( - ) refer 
here to the value of $ e inside and outside of C, respectively. 
Additionally, if Q' represents the component of discharge due 
to $' that is normal to the mine boundary and Q e represents 
the normal component due to <i> e , then in order for flow to be 
continuous, Q e also must jump by an amount 



Q'+-Qe-= -Q< 



(7) 



across the mine boundary. 

A line integral that is a distribution of dipoles oriented 
normal to a line element is referred to as a "line doublet" ele- 
ment. The line doublet has the property that the potential $ is 



discontinuous across the element. Line doublets are placed 
along the boundary C in order to generate the necessary 
discontinuity in potential <t> e . In a similar fashion, the 
necessary discontinuity in the normal discharge vector, Q e , is 
produced by placing line sink elements (line integrals that are 
distributions of sinks) along the boundary C. 

Thus the function 3> e , which can also be interpreted as the 
real part of a complex potential fl e = <£ e 4- i* e , may be written 
as a complex contour integral consisting of doublet and line 
sink elements. The function ft e is the complex potential that is 
valid at z locations outside the area sink; the potential valid at 
z locations inside the area sink is obtained by adding <$' to the 
real part of ft e . 

The complex potential function ft e is given as 



ft e = 




n(6) d<5 



2tt 



In(z-Zi), 



(8) 



where the first term is the complex potential for a line doublet, 
the second term is the complex potential for a line dipole, and 
the second and third terms together constitute the potential for 
a line sink. The condition of continuity of <t> and ¥ along C is 
expressed in equation 8 in terms of density distributions for 
line doublets, X, and line dipoles, /i, respectively. The total 
discharge from the area sink, which is not necessarily known in 
advance is Qo. The integration in equation 8 is performed 
analytically on an area sink boundary C that has been 
discretized as a polygon. The starting point of integration on 
the boundary is zi. 

For a constant infiltration rate 7, the potential function $' 
is given as 



$' = l/2 7 £ 2 



(9) 



-Work cited in footnote 2. 



where £ is a unit length along the boundary of the area sink. 

An area sink may be infiltration-rate specified or head 
specified. In cases were the area sink infiltration rate is known, 
the complex potential (ft, where ft = ft' inside an area sink and 
ft = ft e outside an area sink) can be calculated directly. In 
cases where area sink head <f>* and resistance c are known, 
equation 1 is used to determine the infiltration rate per unit 
area 7. 

The system of equations that results when multiple area 
sink solutions are superimposed, or when area sink solutions 
are superimposed on other analytic element solutions, such as 
wells, is solved directly in MINEFLO using Gauss elimination. 
In an unconfined flow setting, the aquifer thickness is itself the 
piezometric head; thus, the function relating potential, <£, to 
piezometric head, <j>, is nonlinear. In this case, the infiltration 
rate for an area sink that is head and resistance specified is 
calculated iteratively by MINEFLO. 



63 



DROP TEST SIMULATION 



Modeling the distribution of saturated conditions during 
weeks 26 to 41 of block-cave leaching operations consists, 
essentially, of superimposing 3 distributed area sink solutions 
and 15 steady-state well (point source) solutions in order to 
simulate the drop tests conducted in wells 38, 42, and 45. In 
the simulation, just as in the field test, 13 of 14 injection wells 
are specified on the basis of a known fixed head (2,850 ft msl), 
and a condition of zero injection is individually specified for 
each of wells 38, 42, and 45. 



REPRESENTING THE DROP TEST SETTING 

Figures 4 and 5 describe the setting of simulated drop tests 
in plan view and in cross section. The plan-view boundaries of 
the three area sinks used to represent collections of dripping 
and flowing transfer raises during drop tests are shown in 



figure 4. The area sinks are at the undercut level (1,125 ft msl). 
Their boundaries are chosen so that each sink underlies one of 
the three wells (38, 42, and 45). Thus, the three area sinks are 
referred to by these numbers also. 

Figure 5 is a schematic cross section showing components 
of an area sink used to simulate the well 45 drop test. The zone 
of saturated rubbled material above the area sink in figure 5 is 
assumed to be composed of a resistance layer and a source 
aquifer. Flow within the resistance layer, which is a 716-ft 
thickness of rubbled material located between the bottom of 
the injection well slotted interval (1,840 ft msl) and the under- 
cut level (1,125 ft msl), is assumed to be downward only. The 
source aquifer in figure 5 is interpreted as a saturated zone 
located between the top of the resistance layer (1,840 ft msl) 
and the local piezometric surface (2,575 ft msl at well 45). Flow 
in the source aquifer is modeled as two-dimensional and un- 
con fined. 



LU 

u 
< 

r- 
00 

Q 



200 



ouun 






600- 


Cascade 


area \\ \\ \ ^^^\f^ 


400- 




V^ 38 \\ \\ % \ 


200- 


N V 1 

1 x 




n 






u 


1 


i il 



LEGEND 

Injection well collar 

Dripping transfer raise 

Flowing transfer raise 

Haulage drift 
(elev 1.064 ft) 

Undercut drift 
(elev 1.124 ft) 

Distributed area sinks 
at undercut level: 

Area sink 38 

Area sink 45 

Area sink 42 



□ 






400 
DISTANCE, ft 



600 



800 



Figure 4.— Area sink representation of mine infiltration. 



64 




Figure 5.— Schematic cross section of distributed area sink. 



AREA SINK CALIBRATION 

Calibration consists of determining the infiltration rate 
for each area sink, such that the calculated heads at the loca- 
tions of wells 38, 42, and 45 match the measured heads given in 
table 1. 

Rearranging equation 1 to incorporate the definition of 
the resistance parameter c given in equation 2 and writing the 
infiltration rate per unit area y as Q c /A c , where A c is the plan- 
view area of a distributed area sink, gives 

Qc h 



0(Z) = 



(10) 



It follows that for area sinks that are head and resistance 
specified, a calibrated distribution of infiltration is one in 
which the three Q c values chosen (one for each area sink) 
satisfy equation 10 at the locations of the three test wells. 
Because of the uncertainty associated with the permeability 
values obtained from slug tests, a limited amount of manipula- 
tion of resistance values c is permitted during MINEFLO 
calibration runs. By contrast, the value of 0* - 0(z) at the 
location of the test wells is known with much greater certainty. 
The calibration also takes advantage of the fact that the total 
well injection rate and the total mine outflow rate are known 
precisely during the drop tests. 



In equation 10, estimates of resistance layer permeability, k, 
and thickness, h, are available from the drop test results in 
table 1 and from the definitions in figure 5. The plan view area 
A c for each of the three area sinks is laid out in figure 4, and <£* 
is simply the elevation of the undercut level. The difference in 
piezometric elevation, </>*-0(z), between the source aquifer 
and the area sink is available at just one location z above each 
area sink, i.e., at the locations of wells 38, 42, and 45 (see table 
1). 



DISTRIBUTION OF SATURATED CONDITIONS 

The results of calibration are presented in tables 2 and 3. 
The actual and simulated </>* - <t>(z) values at the locations, z, 
of wells 38, 42, and 45 are given in table 2. The discrepancy be- 
tween the two sets of values is small, relative to the total 
thickness of saturated material, and thus equation 8 is con- 
sidered to have been satisfied. In addition, table 2 shows the 
total well injection rate, actual and simulated. Table 3 gives 



65 



the proportional distribution of (total) mine infiltration, i.e., 
367,500 gal/d, used to obtain these results. In the calibrated 
model, area sinks 42 and 45 account for about 95 pet of total 
solution recovery, while area sink 38 accounts for only 5 pet. 
The distribution is consistent with other estimates of plume 
size and shape 5 and with the in-mine survey results of figure 3, 
which show a dense cluster of flowing transfer raises in the 
vicinity of wells 42 and 45 and mainly dripping transfer raises 
spread out in the vicinity of well 38. 

In comparing tables 2 and 3, it is seen that a high infiltra- 
tion rate per unit area of mine workings is associated with an 
overlying test well that has a low-steady-state piezometric 
head; compare, for instance, well 42 and area sink 42. Con- 
versely, a low mine-infiltration rate is associated with a test 
well that has a high piezometric head; compare well 38 and 
area sink 38. This apparent contradiction between piezometric 
condition and mine infiltration is resolved when it is recalled 
that the operational injection wells are head specified in 
MINEFLO, not flow specified. Thus, given the same concen- 
tration of injection wells and the same injection well head 
specification (2,850 ft msl), the observation of a low head in a 
test well above the mine implies the existence of greater 
permeability in the vicinity of the well and therefore lower 
resistance to the downward flow of leach solution. The obser- 
vation of a high head in a test well implies lower permeability 
and greater resistance to downward flow. 

In the case of well and area sink 45, a well head that is 
comparable to well 38 is associated with an area sink infiltra- 



'Swineford, G. Volume of Ore Wetted by In Situ Leaching. Magma Mining 
Co. internal rep., July 1987, 16 pp.; for inf., contact M. Miller, Magma Mining 
Co., San Manuel, AZ. 



tion rate that is about half that of area sink 42. The elevated 
head in well 45 is due to a greater concentration of head- 
specified injection wells (7 of 14 injection wells overlie area 
sink 45). As with all three area sinks, the infiltration rate per 
unit area of area sink 45 is proportional to the overlying 
permeability (see table 1). 

The saturated plume that surrounds the injection wells in 
the rubbled ore mass and in the cascade area can be described 
using the distribution of mine infiltration in table 3. The 
simulated plume is shown in figure 6. The plume boundary is a 



Table 2.— Area sink calibration results, week 41 

Condition Simulated 

Piezometric elevation above the 
undercut level, ft: 

Well 38 1,440 

Well 42 1,004 

Well 45 1,520 

Total injection rate from all 

wells gal/d.. 876,000 



Actual 



1,465 
1,028 
1,450 

875,000 



Table 3.— Area sink outflow distribution 



Area sink Area, Outflow rate, Total outflow 

location' ft* (gal/d)ft 2 rate, gal/d 

38 54,137 0.36 19,438 

42 21,872 7.72 168,849 

45 49,912 3.59 179,216 

Total simu- 

lated outflow 367,503 

'In reference to the overlying test well. 



Percent of 
total 



5 
46 
49 



100 



2,000 



1,500 



LU 

CJ 

< 

r- 
oo 

Q 



1,000 



500- 




( 






LEGEND 




Steady-state well 


( 


Saturated zone 


piezometric contours 




(interval = 100 ft) 


m 


In place ore 


o 


Rubbled material 




Distributed area sinks 




at undercut level: 


LJ 


Area sink 38 


■1 


Area sink 45 


k. 


Area sink 42 



500 1,000 1,500 

DISTANCE, ft 

Figure 6.— Simulated solution plume, week 41. 



66 



zero-head condition in the source aquifer; in the resistance 
layer, it is a vertical free surface extending from the bottom of 
the source aquifer (1,840 ft msl) to the undercut level (1,125 ft 
msl). It is not modeled as a zero flux condition, since figure 2 
indicates that the plume boundary is expanding during almost 



all of the first 41 weeks of leaching operations. The presence of 
a large portion of the plume in the cascade area results from 
the asssumption that there is no permeable connection be- 
tween the cascade area and the underlying haulage drifts. 



PERMEABILITY ESTIMATION USING AREA SINKS 



Consider the inverse of the previous calibration problem. 
The ore permeability is not known in advance, but the 
distribution of infiltration within the mine has been deter- 
mined in an in-mine survey, which precisely measured the 
discharge from individual sump areas on the haulage level or 
possibly even from individual transfer raises. Rearranging 
terms in equation 10, the unknown permeability, k, can be 
written as 



_Q^ 



A c (0*-<Kz)). 



(11) 



Equation 11 is identical to the expression used for 
estimating aquifer recharge due to vertical leakage through a 



deposit, 6 where k/h (the inverse of resistance) is called the 
leakage coefficient, and A c is the area over which vertical 
leakage is diverted to a pumping center. 

If, in addition to the above in-mine survey, steady-state 
drop tests are conducted at well locations carefully chosen to 
provide representative measures of the head distribution in the 
overlying solution plume, then the MINEFLO computer pro- 
gram and the distributed area sink solution can be applied to 
determine a leakage coefficient k/h and a vertical permeability 
k, which satisfies equation 11. The method is appropriate for 
an ongoing block-cave leach setting in which it can be assumed 
that [<t>* - 0(z)]/h measures the average vertical gradient in 
head above the area sink and that <j>* - <j>(z) and Q c /A c are 
constant with respect to time. 



SUMMARY AND CONCLUSIONS 



Distributed area sink solutions developed using the 
analytic element method have been shown to be useful for 
modeling irregular underground mine infiltration areas. 
Together with superimposed analytic element solutions for 
wells and other flow features, area sinks can be used to 
describe the saturated plume that develops above an 
underground mine collection area during block-cave leaching. 

The area sink model aids in describing two important 
hydrologic variables that influence the size and shape of the 
saturated plume: the placement of injection wells above the 
underground solution-collection area and the occurrence of 
permeability layering in the rubbled material overlying the col- 
lection area. 

The area sink and well solutions can be applied via an in- 
verse problem to estimate the leakage coefficient and 



permeability of saturated material overlying the in-mine collec- 
tion area. For instance, the entire in-mine collection area can 
be represented by a single distributed area sink, a single test 
well can be used to estimate the average head in the saturated 
zone above the area sink, and then a single average leakage 
coefficient and permeability can be calculated for the entire 
saturated, rubbled zone. Alternatively, the in-mine collection 
area can be divided into several area sinks based on differences 
in infiltration rate. Steady-state head measurements can be ob- 
tained at one or more locations in the saturated material above 
each area sink, and the permeability of each portion can be 
estimated individually. 



'Walton, W. C. Groundwater Resource Evaluation. McGraw-Hill, 1970, 664 



pp. 



67 



HYDROLOGIC: AN INTELLIGENT INTERFACE 
FOR THE MINEFLO HYDROLOGY MODEL 

By Michael E. Salovich 1 



ABSTRACT 

The U.S. Bureau of Mines has developed an expert system to provide assistance to 
researchers using the Bureau's hydrology computer program MINEFLO. This expert system, 
HydroLogic, is a multifaceted computer program that addresses the various ways in which 
MINEFLO may be used, including the design and analysis of in situ hydrologic operations. 
With tutorials, templates, and diagnostic questions, HydroLogic combines educational 
resources with diagnostic reasoning to create an environment supporting the MINEFLO 
hydrology program. This added support reduces the level of technical difficulty in using 
MINEFLO and thereby opens the model to a wider range of possible users. As a conse- 
quence, MINEFLO's effectiveness as a hydrologic tool for the mining industry is enhanced. 



INTRODUCTION 



The design and analysis of in situ systems require an 
understanding of the various factors that interact and define 
the hydrology of the system. This is a complicated task 
because, over time, hydrologic parameters, such as aquifer 
permeability, solution injection rates, well locations, and solu- 
tion recovery rates, interact and change. An impermeable layer 
may form, the leachant solution may alter the permeability of 
the constituent ore deposit, or the occurrence of fracture fault 
lines may increase as subsidence occurs during block-cave 
leaching. These changes, in turn, call for a response in mining 
operations. Injection rates may have to be raised, wells may 
have to be repositioned and drilled deeper, the chemistry of the 
leachant solution may have to be remixed, or the in situ opera- 
tion itself may have to be reevaluated and discontinued. In 
other words, this type of analysis of in situ systems can be ap- 
proached as a study of parameters — hydrologic parameters 
that are identified and manipulated so that their interaction 
and effects are understood. In situ mining is a dynamic proc- 
ess. Understanding these factors and being able to anticipate 
their interaction is vital for efficient in situ operation. In light 
of this, the Bureau of Mines has developed two computer pro- 
grams that work together in the analysis of these hydrologic 
factors: MINEFLO and HydroLogic. These programs have 



'Computer programmer analyst, Twin Cities Research Center, U.S. Bureau of 
Mines, Minneapolis, MN. 



been developed to assist the mining industry in the planning 
and development of in situ mining operations, as part of the 
Bureau's effort to increase minerals extraction efficiency. 

MINEFLO is a hydrologic simulator that provides de- 
tailed and general analyses of hydrologic systems. It can pro- 
vide information about aquifer permeability, well discharge 
and collection rates, leachant streamline direction and veloci- 
ty, changing head pressures, and the effects of fractures and 
impermeable zones. Although this hydrologic simulator is 
robust and versatile, prior understanding of the hydrologic 
terms and methods it employs is required. With the goal of 
making MINEFLO a useful tool for the mining industry, the 
Bureau developed HydroLogic — a companion computer pro- 
gram designed to help researchers use MINEFLO. 

HydroLogic, the focus of this paper, is a multifaceted 
computer program that addresses the various ways in which 
MINEFLO can be used. With diagnostic questions, tutorials, 
and templates, HydroLogic combines educational resources 
with diagnostic reasoning to create an "expert support environ- 
ment" for the MINEFLO hydrology model. In other words, 
HydroLogic functions as an intelligent interface for 
MINEFLO. In the sense that expert systems are computer pro- 
grams that attempt to emulate the thoughts and actions of an 
expert in a particular field of knowledge, HydroLogic is an ex- 
pert system that provides expert guidance to those using 
MINEFLO. 



68 



ENVIRONMENT 



HydroLogic was developed in HyperTalk, the script 
language of HyperCard, for use on Apple Macintosh com- 
puters. 2 HyperCard provides an excellent environment for the 
implementation of HydroLogic because it offers "hyper- 
media" capabilities (i.e., interactive graphics combined with 
text) and the flexibility to be linked to other computer pro- 
grams and data bases. HyperCard's advanced graphics tools 
enable the HydroLogic expert system to present different com- 
binations of pictures, diagrams, and text with "cursor 
sensitive" areas for simple communication. Figure 1 shows the 
main menu of HydroLogic. Each rectangle in this picture is 
designed to be a "graphic button," sensitive to the cursor. 
When a button is selected, the system interprets it as a com- 
mand and responds accordingly. With this method, users can 
navigate HydroLogic's menus efficiently by communicating 
commands on a graphic level. 

HyperCard also has the ability to act as a central control 
point for accessing other computer files and applications. 
With its "open" command, it can initiate other computer pro- 



reference to specific products does not imply endorsement by the Bureau of 
Mines. 



grams, and with its "read" and "write" commands, it can ac- 
cess and manipulate any other computer files residing in 
memory. These commands allow HydroLogic to read and 
manipulate MINEFLO's data sets or to start the program 
itself. This creates a workstation environment that eliminates 
the awkwardness of having to switch back and forth between 
the consulting program and the intended application. 

Unlike many computer programs, HydroLogic is not an 
isolated system; it provides a dynamic environment that can 
interact with any resource on the computer. This is important 
because HyperTalk's control structures are limited in creating 
systems that involve complex recursion or symbol manipula- 
tion. Future development of HydroLogic, then, may be ex- 
tended to include additional artificial intelligence languages or 
systems, such as PROLOG or LISP. 

Versions of the MINEFLO hydrology model exist on the 
MicroVax, Harris, and Macintosh computers. At this time, 
versions of the Hydrologic system operate only on the Macin- 
tosh II, Macintosh SE, and Macintosh Plus. Because of the 
size of the HyperCard code and the possibility of creating large 
data sets with MINEFLO, a hard disk is necessary for op- 
timum performance. 



rfliihnrihiiiflMniin 



HydroLogic 



1:57 | l 



Quit I 



^jj|||w 





Mineflo 




Tutorials 


Templates 


Diagnostics 






mmmmmmmmm 



Figure 1.— HydroLogic's main menu, composed of graphic buttons. 



69 



DESIGN 



The HydroLogic system is composed of three major sec- 
tions: Diagnostics, Tutorials, and Templates. Only the 
Diagnostics section employs the diagnostic queries familiar in 
most expert systems. The Tutorials and Templates sections are 
educational resources that provide important information 
about the methodology and terminology used by the 
MINEFLO program. This combination of educational 
resources with diagnostic analysis enables HydroLogic to pro- 
vide a broad base of support to those using the MINEFLO 
hydrology model. 

DIAGNOSTICS 

Most diagnostic systems require a small problem domain 
because of the way they capture and apply knowledge. This is 
done commonly through production rules — simple "IF . . . 
THEN" statements. These rules are carefully designed and 
structured so that they recreate the various steps of reasoning 
an expert may take in reaching a decision. Since all the mental 
steps involved in analyzing a problem must be explicitly 
represented in production rules, a problem must be sufficiently 
limited in its complexity to be addressed effectively by a 
diagnostic system. Otherwise, the diagnostic system would be 
too complex to design properly. 

Additionally, diagnostic systems defined by production 
rules tend to be stiff and brittle. This is because production 
rules are predefined and their coherency can easily break down 
in the face of unforeseen situations. Since most of the produc- 
tion rules are interrelated, it is very difficult to make even the 
smallest changes to a system without a total reconfiguration. 
Therefore, the problem domain addressed by a diagnostic 
system should be sufficiently limited in complexity for reasons 
of effectiveness and efficiency. 

The task of HydroLogic, as stated before, is to provide 
assistance to people using MINEFLO. The ideal expert, the 
model for the system, is someone who knows how to operate 
and apply MINEFLO in various types of situations. This, 
however, is a formidable task. The different combinations of 
MINEFLO input errors, user questions, and modeling prob- 
lems are numerous and create a large problem space. This task 
is too large and complex to be accurately and efficiently 
represented by a single structure of production rules. 
However, subsets of this problem domain (i.e., specific topics 
concerning input parameters, error diagnosis of streamline 
generation, etc.) are less complex and therefore are viable sub- 
jects for smaller diagnostic systems. The large problem domain 
confronting HydroLogic could therefore be reduced to a col- 
lection of smaller and relatively simple subset units. 

This translation of a large problem space into a collection 
of smaller units was the key for the development of 
HydroLogic. This method of "distributed problem spaces," 
developed during the research, opened the way for creating ex- 
pert systems for large or inexact problem domains. It is based 
upon the observation that a large or partially defined problem 
space can be mapped to a collection of smaller well-defined 
subsets. These smaller subset units are logically independent 
and highly specialized and therefore can be easily represented 
through production rules. Thus, an expert system, composed 
of these diagnostic subsets, can be created for a large or par- 
tially defined problem space. 

HydroLogic, then, can be viewed as a collection of small 
diagnostic systems. These subsets are highly specialized, and 
some border on being trivial, but they are related under the 
common theme of being a resource for the MINEFLO 



hydrology model. Figure 2 shows how the HydroLogic system 
enables users to select one of the diagnostic subtopics for con- 
sultation. Figure 3 shows a partial decision tree, depicting the 
typical scope and complexity of a diagnostic subtopic. These 
diagnostic subtopics use forward chaining to proceed from an 
initial (output problem) state to a goal (input revision) state. 
The initial problem state shown in figure 3 is the condition 
where the user of MINEFLO cannot start MINEFLO's flow 
streamlines. The rules in this case consist of all the required in- 
put specifications for generating streamlines using MINEFLO. 
The goal (revision) state is a condition in which the input error 
that prevented streamlines from starting is identified for the 
user. 

If the logical structure formed by a set of production rules 
can be viewed as a decision tree, these subsets can be con- 
sidered branches of a larger or, perhaps, partially defined tree. 
Therefore, the method of distributed problem spaces is a 
means of adding modularity to the design of a diagnostic 
system. An expert system, then, designed in this manner en- 
joys the same benefits as other modular programs: ease of 
design, maintenance, and modification. Small subsets are less 
complex and therefore are easier to create and manage. 
Together the subsets can create an extremely robust and flexi- 
ble system. In addition, because the main problem is expressed 
in subtopics, users enjoy the benefit of having greater control 
over the system. One of the difficulties with expert systems is 
that they tend to interrogate users with a litany of seemingly 
trivial or inappropriate questions. With a modular program, 
users may select the topic of inquiry and thereby constrain the 
flow of questions. 

As stated before, HydroLogic incorporates educational 
resources with the Diagnostics section. This is done because 
certain types of information are more easily and effectively 
communicated through references rather than through 
diagnostic analysis. For example, if a researcher needed to 
locate a menu of commands inside the MINEFLO program, it 
is apparent that viewing a structured diagram of MINEFLO's 
menu hierarchy would be more efficient than submitting to a 
diagnostic question-answer session. Therefore, for reasons of 
efficiency, the HydroLogic system was designed to include, in 
addition to its Diagnostics section, sections containing educa- 
tional tutorials and templates. 

TUTORIALS 

HydroLogic's Tutorials section is a collection of interac- 
tive help files containing explanations and definitions to assist 
MINEFLO users with basic definitions and terminology 
associated with in situ hydrology. For example, tutorials have 
been developed to describe the analytic element methodology 
of the MINEFLO program. Specific tutorials exist for each of 
the analytic element solutions used in the program for model- 
ing hydrologic features such as wells, permeability zones, bar- 
riers, fractures, etc. They contain information ranging from 
MINEFLO's mathematics to the various steps needed for the 
creation of a three-dimensional profile of an aquifer. The 
tutorials are interactive and easy to use, employing graphic 
buttons and other hypermedia features. 

TEMPLATES 

HydroLogic's Templates section contains well-defined ex- 
amples of MINEFLO data sets. (MINEFLO uses data sets as a 
permanent means of recording and storing information about 



70 



Diagnostics 



HydroLogic 



CCQ0@[^[J)QQ(BDQ fflDB" [iQD[p n 



discharge rates are too high 


O 


discharge rates are too loin 




head ualues are too high 




head ualues are too low 




streamlines are jagged 




streamlines stop prematurely 




streamlines don't start 




streamlines are uery slow 




streamlines stop because head is zero 


O 




Figure 2.— HydroLogic's diagnostic subsets displayed in scroll box. 



Rre you starting 
streamlines interactiuely? 



Ves 



flduice No. 1 
Continue? 




No 



No 

Return 




Ves 



Haue you specified 
any starting points? 



v No 
flduice No. 2, Return 




Do you know the streamline 
termination condition? 




Ves 



No 



Figure 3.— Partial decision tree for diagnostic subtopic 
'streamlines don't start." 



a particular hydrologic setting.) Each template is designed to 
convey important information about how to apply MINEFLO 
to a particular type of problem. Essentially, the templates are a 
way of imparting to MINEFLO users the Bureau's experience 
and expertise in identifying hydrologic concerns that are com- 
monly associated with in situ mining activity. In this sense, 
they serve as starting points for researchers creating new data 
sets for the MINEFLO program. Users profit by running the 
MINEFLO program using these data sets and by manipulating 
the various input parameters. As researchers become more 
familiar with the program, they can build on the data sets and 
customize them, thus progressively generating their own in- 
sights and understandings about their particular hydrologic 
setting. In addition, the templates provide concrete examples 
of correct and acceptable input parameter definitions for the 
MINEFLO program. 

Figure 4 shows an example of a template title card. Each 
card contains a title, a key of important features, and a 
graphic image generated by the MINEFLO hydrology model. 
The template shown in figure 4 is a fully defined data set used 
for modeling a single leachant injection well drilled above an 
underground mine void. It contains the basic hydrologic 
features characteristic of a method of in situ leaching called 
block-cave leaching. The key features of the data set are iden- 
tified on the template, and users have the option of looking at 
additional notes, running the MINEFLO program with this 
data set, or copying and modifying it by adding more wells, 
changing the shape and location of the underlying mine, or 
adding other features. 



71 




Figure 4.— Example of a template title card. 



DISCUSSION 



The primary purpose of HydroLogic and MINEFLO is to 
provide the mining industry with an effective tool for the 
design and analysis of in situ hydrologic systems. The 
MINEFLO hydrology model offers the capability to analyze 
the complex interaction of several types of hydrologic 
elements. HydroLogic functions as a companion program that 
provides information and support regarding MINEFLO's ter- 
minology and methodology. The measure of success of 
HydroLogic is its usefulness in providing support to 
MINEFLO. Although the basic steps have been taken in 
establishing diagnostic modules, tutorials, and templates for 
fundamental topics concerning MINEFLO, it is recognized 



that further additions are necessary for the Hydrologic system 
to be considered comprehensive. As the Bureau's experience 
with the MINEFLO hydrology model grows, further develop- 
ment is planned for the HydroLogic system. It is also recog- 
nized that there are different types of computers used in the 
mining industry and that versions of MINEFLO and 
HydroLogic that operate on these computers should be con- 
sidered. It is planned that, as other computer systems develop 
the necessary graphic and hypermedia capabilities required by 
these programs, versions of MINEFLO and HydroLogic will 
be developed for these systems. 



SUMMARY AND CONCLUSIONS 



In summary, HydroLogic contains three sections: 
Diagnostics, Tutorials, and Templates. Together they create a 
support environment for the MINEFLO hydrology computer 
program. Although they do not form a complete resource, 
they provide important information and perspective about 
MINEFLO's capabilities and operation. With the additional 



support of HydroLogic, the task of learning how to operate 
MINEFLO is simplified. This added simplification, in turn, 
translates into greater productivity because it opens 
MINEFLO to a larger number of possible users and adds to 
the depth and quality of research performed with the model. 



72 



BIBLIOGRAPHY 

Dreyfus, H. L., and S. E. Dreyfus. Mind Over Machine. Free Press, Rich, E. Artificial Intelligence. McGraw-Hill, 1983, 436 pp. 

1986, 231 pp. Sowa, J. F. Conceptual Structures. Addison-Wesley, 1984, 481 pp. 

Nilsson, N. J. Principles of Artificial Intelligence. Morgan Kauf- Waterman, D. A. A Guide to Expert Systems. Addison-Wesley, 

mann, 1980, 476 pp. 1986, 419 pp. 

Reboh, R., J. Reiter, and J. Gaschnig. Development of a Winston, P. H. Artificial Intelligence. Addison-Wesley, 1984, 524 

Knowledge-Based Interface to a Hydrologic Simulation Program. pp. 
SRI, 1982, 110 pp. 



73 



PREDICTING AND MONITORING LEACH SOLUTION FLOW 
WITH GEOPHYSICAL TECHNIQUES 

By Daryl R. Tweeton, 1 Calvin L. Cumerlato, 2 Jay C. Hanson, 2 and Harland L. Kuhlman 3 



ABSTRACT 

The U.S. Bureau of Mines is conducting research to develop improved methods for 
predicting and monitoring the flow of leach solution during in situ mining. Potential benefits 
include more reliable assessment of leaching feasibility, higher metal recovery through better 
leach solution distribution, and greater confidence by regulatory agencies that leach solution 
will not escape from the mine. 

The ability of seismic cross-hole tomography to detect fractured zones and saturated 
areas is being field tested for applications in predicting flow patterns and in monitoring leach 
solution injected above the water table. A tomographic analysis computer program, BOM- 
TOM, was written with special options for geophysical applications. It was first used with 
seismic refraction data for detecting fractured zones in a quarry. Tomographic analysis of 
seismic cross-hole travel-time data detected a mound in the water table made by injecting 
water between the source and receiver boreholes. Research to improve the reliability of 
tomographic reconstructions is continuing. Electromagnetic methods for determining where 
high-conductivity leach solution has replaced ground water are also being evaluated. 
Preliminary computer simulations indicate that surface-to-borehole time-domain elec- 
tromagnetic induction is promising. 



INTRODUCTION 



IMPORTANCE OF PREDICTING AND 
MONITORING FLOW 

Reliably predicting and monitoring the flow of leach solu- 
tion is important in planning and operating an in situ mine. 
Predicting flow patterns is critical in assessing the feasibility of 
mining; if most of the flow is in a few large fractures, much of 
the ore may remain unleached. If mining is feasible, predicting 
and verifying flow patterns can aid in planning well spacings, 
selecting depths at which to perforate casings, and choosing 
pumping rates to provide the best leaching coverage of the ore. 
Monitoring the flow in a pilot-scale study can help in deciding 
if well spacing should be changed in a commercial operation. 
Better distribution of leach solution will result in its contacting 
more of the ore, which will increase recovery of the ore 
mineral. 

Safeguarding the ground water resources near an in situ 
leach mining operation is a responsibility shared by the 
operator and regulatory agencies. Obtaining permits from 
regulatory agencies is one of the critical steps in starting an in 
situ mine. Improved methods of predicting and monitoring 



'Research physicist. 

2 Geophysicist. 

'Engineering technician. 

Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN. 



flow could help mining companies control leaching solutions 
and assure regulatory agencies that these solutions will not 
escape from the well field. 



ADVANTAGES OF GEOPHYSICAL METHODS 

Bureau of Mines researchers believe that certain cross- 
hole or surface-to-borehole geophysical techniques could pro- 
vide important advantages over present methods of predicting 
and monitoring flow. The present method for finding prob- 
able flow paths, such as fractured zones, is a combination of 
borehole geophysics and examination of cores. The volume in- 
vestigated is limited to the region close to the boreholes. Im- 
portant geologic features could be missed unless the boreholes 
are closely spaced. The present method of monitoring is to 
sample monitor wells. However, the flow of leach solution 
through an ore body is not uniform, especially in fractured 
hard-rock deposits, such as copper and most precious-metal 
ores. The inhomogeneous flow pattern can decrease the 
reliability of monitor wells for locating leach solution. Placing 
monitor wells sufficiently close together so that regulatory 
agencies are confident that leach solution cannot escape 
undetected can be expensive. At the 600-m depth at which in 
situ copper mining can occur, a monitor well costs several tens 
of thousands of dollars. 



74 



Cross-hole geophysical methods with tomographic data 
analysis and surface-to-borehole methods can examine the 
region between boreholes. Thus, an important geologic feature 
is less likely to be missed. Used before injection, these methods 
could indicate probable flow paths. Comparing measurements 
made before and after injection could locate leach solution. 
An additional economic advantage of using geophysical 
systems over coring and sampling monitor wells is that the 
systems can be reused, which decreases the overall cost over 
the life of the mine. 



PRIOR RESEARCH 

Bureau contract research in 1979 (I) 4 indicated that 
neither surface four-terminal resistivity nor controlled-source 
audio-magnetotellurics (CSAMT) reliably detected carbonate 
leach solution at a depth of 80 m at an in situ uranium mine in 
Wyoming. The surface resistivity measurements were 
hampered by the thinness of the leached zone compared with 
its depth from the surface. The CSAMT system suffered from 
interference, such as that from powerlines. However, 
technology has improved since that time, so that failure of the 
surface resistivity and CSAMT systems does not necessarily 
mean that those techniques would fail with modern 
geophysical equipment. 

Tomography (explained in the section "Seismic Cross- 
Hole Tomography") has been applied to geophysical investiga- 
tions by a number of researchers. Ramirez (2) reported the ap- 
plication of tomography to electromagnetic (5- to 40-MHz) at- 



tenuation cross-hole data with borehole separations up to 30 
m. Dines (3) analyzed electromagnetic (50-MHz) cross-hole 
data, measured both attenuation and velocity, and found at- 
tenuation to be more diagnostic. Applications to seismic 
reflection data were made by Chiu (4) and Bishop (5). 
Analyses of seismic cross-hole data were performed by 
Albright (6), Gustavsson (7), Peterson (8), and Peterson (9). 
Foss (10) applied tomography to detecting coal mine hazards 
with ground-penetrating radar (GPR) but reported that 
tomography did not conclusively show the clay vein of in- 
terest. Werniuk (11) described research being coordinated by 
Paul Young of Queen's University, applying tomographic im- 
aging to mining-induced seismicity and rock burst phenomena. 
A helpful summary of the advantages and disadvantages of the 
various tomographic mathematical methods was given by 
Worthington (12). 



RECENT AND CURRENT BUREAU RESEARCH 

The Bureau's current emphasis is on seismic cross-hole 
tomography to predict and monitor the flow of leach solution. 
Preliminary experiments have been conducted to test the abili- 
ty of this method to locate a curved water surface, simulating 
locating leach solution injected above the water table. In addi- 
tion, seismic tomography has been used for detecting fractured 
zones in a quarry and in building stones. The Bureau is also 
evaluating electromagnetic methods to monitor leach solu- 
tions, which are described in more detail in a subsequent sec- 
tion. 



SEISMIC CROSS-HOLE TOMOGRAPHY 



BACKGROUND 

The seismic cross-hole method consists of transmitting 
seismic waves from one borehole to another and measuring the 
corresponding velocity and/or attenuation. Measuring 
velocities is easier and the interpretation is usually easier. The 
seismic velocity is lower and the attenuation is greater in zones 
of fractures and broken rock than in solid rock. The seismic 
compressional wave velocity increases with saturation. In 
Bureau laboratory experiments using competent cores from 
various rock types, the velocity was typically about 10 pet 
greater in saturated rock than in dry rock (13). Bureau re- 
searchers expect that there will not be a measurable difference 
between the velocity in rock saturated with leach solution and 
that in rock saturated with ground water. 

The general procedures involved in tomography have 
been described elsewhere (3, 8-9, 12, 14-15). Only a brief 
discussion, as it relates to this special application, will be 
given. Tomographic reconstruction as applied to cross-hole 
data is a mathematical process for constructing a two- 
dimensional representation of a field, such as seismic velocity 
between two boreholes. Figure 1 shows a simplified diagram of 
the source (transmitter) and receiver locations. In practice, 
there would be many source and receiver locations, so the 
region between the boreholes would be crossed by many ray 
paths. One method of collecting tomographic data consists of 
setting the source at one position, stepping the receiver 
through its positions, then moving the source to its next posi- 
tion and repeating the series of receiver positions. This pro- 



"Italic numbers in parentheses refer to items in the list of references at the end 
of this paper. 



cedure is repeated until all of the desired ray paths have been 
generated. Alternatively, the receiver position can be fixed, 
while the source position is varied. If the borehole is filled with 
fluid so that coupling is adequate without clamping the source 
to the borehole wall, the source can be moved continuously 
and fired at timed intervals. 

Tomographic analysis of cross-hole seismic travel times 
provides a two-dimensional picture of the distribution of 
seismic velocities between the source and receiver boreholes. 
The positions of zones of fractures and leach solution can then 
be inferred from the seismic velocities. Albright (6) showed 
that seismic cross-hole tomography can indicate geologic 
structure, including zones of fractures. Research is being con- 
ducted to determine the reliability of using the method to show 
the location of leach solution. The method appears promising 
for detecting leach solution when the ore is unsaturated before 
injection. Thus, the seismic method may be applicable for 
detecting likely flow paths above or below the water table and 
for locating leach solution injected above the water table. 

Compared with surface techniques, seismic cross-hole 
methods offer higher resolution, higher sensitivity to features 
in the area of interest, and lower sensitivity to features outside 
the area of interest. However, boreholes are expensive; to be 
practical, the cross-hole methods must use boreholes that are 
needed for other purposes, such as injection or monitor wells. 

Compared with other cross-hole techniques, the seismic 
method has several advantages. Seismic waves can easily 
penetrate several hundred meters and propagate through cas- 
ings. The equipment is relatively inexpensive. For example, an 
air gun source costs about $12,000. A disadvantage is that this 
method cannot distinguish between ground water and leach 
solution. 



75 



Source 
well 




Injection 
wells 



Receiver 
well 




''....V-lV-'-/— -:* 

^ / / \ V 

^ \ 

2 v 



7 







Figure 1.— Source (transmitter) and receiver locations in boreholes. 



76 



TOMOGRAPHIC ANALYSIS COMPUTER 
PROGRAM 

Bureau researchers wrote the tomographic analysis com- 
puter program BOMTOM (Bureau of Mines tomography) 5 
because available computer programs did not contain options 
needed for the most effective analysis of cross-hole data. To 
perform tomographic analysis, the region between the 
boreholes is mathematically divided into small cells called pix- 
els. The tomographic reconstruction assigns a velocity to each 
pixel to provide the best fit to the data. Tomographic analysis 
of cross-hole data without constraints does not yield a unique 
reconstruction, even when the number of measurements ex- 
ceeds the number of pixels. Thus, a unique reconstruction can- 
not be obtained by increasing the number of cross-hole 
measurements. Cross-hole data can be fit equally well by many 
different reconstructions, so appropriate mathematical con- 
straints must be applied to help select the correct one. A 
tomographic program providing a test for nonuniqueness and 
constraints to counteract nonuniqueness was needed. 
Available programs did not provide such a test or constraints 
appropriate for in situ mining applications. 

BOMTOM provides a test for uniqueness and constraints 
to help obtain unique reconstructions. It is easy to run on per- 
sonal computers commonly used by mining and geophysical 
companies. Options can be selected interactively. BOMTOM 
uses the simultaneous iterative reconstruction technique 
(SIRT) with straight-ray paths. The adequacy of straight-ray 
paths for in situ mining applications will be examined in future 
research. A curved-ray program with suitable constraints is be- 
ing developed for use in situations in which velocity contrasts 
are large and bending of rays is significant. 

BOMTOM allows the user to test uniqueness by varying 
the pattern of the initial velocity guesses used to start the 
iterative solution procedure. A unique reconstruction is in- 
dependent of the initial velocity guesses. BOMTOM provides 
the following options for constraints: 

1. Maximum and minimum calculated velocities; 

2. Known, fixed velocities in boreholes, as from sonic 
logs; 

3. Horizontal layers near the top and/or bottom of the in- 
vestigated area, in which seismic velocity does not vary with 
horizontal position; 

4. Smoothing, in which the calculated velocity in a pixel is 
influenced by the velocities in neighboring pixels. 



A Bureau report (16) describes BOMTOM and results of 
testing the effectiveness of various combinations of constraints 
in obtaining unique reconstructions when simulating in situ 
mining. Calculations were performed with synthetic data of 
the type expected from in situ mining above the water table, 
with a dip in the leach solution level between injection wells. 
The seismic velocity distribution was modeled by attributing a 
velocity increase of 10 pet to the leach solution. The simula- 
tions generated a high-velocity artifact near the dip between 
the wells. (A tomographic artifact is an error in the calculated 
velocity for the corresponding pixels.) The artifact was re- 
duced by the constraint that velocity was independent of 
horizontal position near the top and bottom of the pixel grid. 
This can be a realistic constraint when the ore at the top of the 
grid is dry and the ore at the bottom is saturated, and there are 
no other factors (such as nonhorizontal bedding) causing a 
significant change with horizontal position near the top and 
bottom of the grid. Applying the additional constraint of fix- 
ing the velocities in the boreholes at known values reduced the 
artifact slightly more. The reconstruction improved signifi- 
cantly when the upper velocity limit was close to the highest 
velocity in the model. However, the appropriate upper limit 
may not be well known for field data. The simulations 
demonstrated the need to consider nonuniqueness when per- 
forming such calculations and the desirability of surrounding 
the investigated region by an area of known or constrained 
velocities. One way to impose constraints at the sides of the 
region is to measure the seismic velocities at intervals in the 
boreholes with sonic logs and fix the corresponding velocities 
to those values during the tomographic data fitting. 

EQUIPMENT DEVELOPMENT 

The Bureau developed an improved well-locking 
geophone receiver system. It provides a better signal-to-noise 
ratio than commercial seismic receiver units that the Bureau 
has leased. The improved signal-to-noise ratio allows a lower 
strength source to be used, thereby reducing the risk of damag- 
ing the well casing. It is smaller, easier to use, and less expen- 
sive than most commercial units. It will be described in a later 
publication after clarification of patent status. 

A truck-mounted field system is being prepared for re- 
cording field data. It includes a generator, an air compressor, 
winches, and all equipment needed to make field ar- 
rangements. Field tests will be conducted in the type of rock 
and at the depths typical of in situ copper mining. 



DETECTING FRACTURES WITH SEISMIC TOMOGRAPHY 



DETECTING FRACTURED ZONES IN A QUARRY 

BOMTOM was used for detecting fractured zones in the 
Rockwell lime quarry near Rockwood, WI, by analyzing 
seismic refraction data (77). The general principles behind 
detecting fractures in quarries and detecting fractures at an in 
situ mine are identical; namely, the seismic velocity decreases 
in fractured zones (18). Thus, examining the application of 
tomography to seismic refraction data can indicate its promise 
for analyzing seismic cross-hole data to detect fractured zones 
in in situ mines. 



'Source and executable codes in FORTRAN on 5'/>- and 3'/2-in IBM- 
compatible diskettes are available upon request from D. R. Tweeton, Twin Cities 
Research Center, Bureau of Mines, Minneapolis, MN 55417. 



Bureau researchers needed a fracture detection technique 
to evaluate the success of blast design changes in reducing pit- 
wall damage. Standard seismic methods were inadequate, so 
seismic refraction tomography was employed. Each survey 
consisted of a series of refraction fan shots in which a line of 
geophones was arranged parallel to and offset from a line of 
shot points (fig. 2). Refraction tomographic surveys consisted 
of 24 shot points and 24 geophone stations, yielding 576 ray 
paths per survey. Seismic travel time data were used for imag- 
ing the seismic velocity distribution on the refracting rock 
layer. 

Tomographic reconstructions were performed with BOM- 
TOM. Travel times were accurate to ±0.05 ms. Velocity con- 
straints consisted of high and low limits and the use of a 
uniform initial velocity guess. Other optional parameters in- 
cluded the use of a delay time to correct for seismic wave travel 



77 



Planes of travel paths 
through surface layer 




Refracting layer 



G Geophone station 
S Shot point 



Figure 2.— Conceptual drawing of ray-path geometry for series of four refraction fan shots. 



time through the surface layer and the use of a weighted 
smoothing routine after BOMTOM arrived at a final solution. 

A production blast design was modified so that half of the 
last row of shot holes were loaded with the quarry's standard 
explosive load, while the remaining holes were loaded with a 
reduced-diameter explosive column in the stemming zone (fig. 
3). This was done to reduce cratering while promoting shearing 
of the rock. 

The results of the preblast and postblast surveys are 
shown in figures 3 and 4, respectively. The size of each pixel is 
0.5 m. The most obvious feature in the preblast tomogram is a 
low-velocity trend from the lower right to near the upper left 
corner. Since a predominant joint set was found in the quarry 
with an average strike of N 26° W, it appears reasonable that 
this feature is the seismic velocity signature of a similar joint 
set intersecting the shallow refracting layer. The low-velocity 
zone in the upper right corner may be the expression of 
another joint set or it may be a zone of fracturing from a 
previous blast. 

Comparing the preblast and postblast tomograms (fig. 5) 
shows that the velocity field of the refracting layer has 
changed. In the upper left and upper center areas, closest to 
the standard blast design, zones of relatively high velocity 
before the blast now appear as low-velocity zones. The low- 
velocity feature interpreted as jointing in the preblast 
tomogram is also more pronounced, especially in the lower 
right area. In addition, the high-velocity area nearest the 
shotholes with the modified blast design has remained about 
the same. 

It appears that the changes made in the blast design in the 
five right-hand shotholes have contributed to a reduction of 
fracturing in the rock left standing, while the rock nearest the 



shotholes with the standard load has been extensively dam- 
aged. It is possible, however, that much of the energy pro- 
duced by the entire blast propagated into or was channeled 
through the preexisting jointed area, causing some additional 
fracturing, which makes this zone more pronounced in the 
postblast tomogram. In either case, the results of the seismic 
refraction tomographic analysis are consistent with known 
features of the rock. 

There are two possible sources of error that users of this 
method should consider. One source of error is the uniform 
delay time for all ray paths, since lateral velocity variations 
were present in the surface layer as a result of the blast under 
investigation and subdrilling from the previous bench. The 
other source of error is the use of straight-ray paths. The 
velocity contrast in quarries is large enough to make curved- 
ray analysis desirable. Work is in progress to develop a curved- 
ray program with BOMTOM's features. 

The methods applied during this study were successful in 
identifying major velocity trends in shallow refracting rock 
layers. The trends can be related with confidence to jointing 
and zones of intense fracturing caused by mine blasting. Fur- 
thermore, refraction tomography may hold great potential for 
future widespread application as a powerful and economic 
tool for mine planning and blasting design. 

DETECTING FRACTURES IN BUILDING STONES 

BOMTOM has been used by a private company for ex- 
amining building stones by analyzing seismic travel times for 
two pairs of faces. Fractured zones that were not visible from 
the surface were detected with seismic tomography. 



78 




m 



Receiver 
positions 



-21 m 
Source 
positions 



-I 



KEY 



3.00-3.25 km/s 
3.25-3.50 km/s 
3.50-3.75 km/s 
3.75-4.00 km/s 
4.00-4.25 km/s 
4.25-4.50 km/s 
4.50-4.75 km/s 
4.75-5.00 km/s 

standard blasthole load 
modified blasthole load 



Figure 3.— Preblast tomogram and location of blast design variations. 




m 






Receiver 
positions 



21 m 

Source 
positions 



N 



KEY 

3.00-3.25 km/s 
3.25-3.50 km/s 
3.50-3.75 km/s 
3.75-4.00 km/s 
4.00-4.25 km/s 
4.25-4.50 km/s 
4.50-4.75 km/s 
4.75-5.00 km/s 

standard blasthole load 
modified blasthole load 



Figure 4.— Postblast tomogram. 



79 




-0 m 



N 1 



KEY 



Receiver 
positions 



| -2.0 to -1.5 km/s 

I -1.5 to -1.0 km/s 

| -1.0 to -0.5 km/s 

-0.5 to 0.0 km/s 

0.0 to 0.5 km/s 

0.5 to 1.0 km/s 

11.0 to 1.5 km/s 
1.5 to 2.0 km/s 

• standard blasthote load 

• modified blasthole load 



21 m 

Source 
positions 



Figure 5. — Preblast velocities minus postblast velocities. 



LOCATING CURVED WATER SURFACE WITH SEISMIC TOMOGRAPHY 



PRELIMINARY TESTS AT BUREAU'S SITE 

Preliminary experiments to evaluate the use of seismic 
travel times to locate leach solution injected above the water 
table were conducted at the Bureau's on-site geophysical test 
facility. Water was injected instead of leach solution because 
the change in travel times depends on physically wetting the 
rock rather than on the chemical properties of the solution. 
The experiment was conducted to help answer two questions: 

1. Does the increase in the velocity of seismic waves in 
saturated rock observed in the laboratory (13) require the 
thorough drying and saturating used in those experiments, or 
will it also occur under field conditions? 

2. If travel times do decrease as the rock becomes 
saturated, how well can tomographic analysis locate the 
regions where the rock changed from dry to wet? 

Tests were conducted by transmitting a seismic signal be- 
tween boreholes 4.2 m apart in fractured limestone. A mound 
in the water table was created by injecting water into a 
borehole between the source and receiver boreholes. The 
seismic source was an air gun. The receiver was a wall-locking 
triaxial borehole geophone. The source and receiver positions 
were 3.7 to 9.1 m below the surface, in steps of 0.3 m. 

Data were first collected when no water was being in- 
jected. The water level in all boreholes was 6.1 m below the 
surface. After a tomographic data set was collected, water was 
injected into the middle well at about 40 L/min. This injection 
raised the water levels in the source, injection, and receiver 



wells to 4.9, 4.2, and 5.3 m, respectively. A curved water sur- 
face was desired to test the ability of the tomographic 
reconstruction to show the shape of the top of the water sur- 
face between the source and receiver wells. A problem 
prevented collection of a complete tomographic data set. The 
air gun repeatedly became stuck in the uncased limestone 
borehole, so about one-fourth of the desired ray paths near the 
bottom of the grid could not be used. 

The data 6 were analyzed with BOMTOM, using 
smoothing. The reconstruction using preinjection data showed 
high-velocity zones corresponding to water at the expected 
level, but there were also high-velocity zones above the water. 
The reconstruction using data taken during injection showed 
the mound in the water where it should be, but mathematical 
artifacts and effects not related to the mound were present. 
There were at least two problems interfering with the 
tomographic analysis. First, the coverage was not complete. 
Second, the maximum calculated velocity was three times as 
great as the minimum calculated velocity, so the use of straight 
rays could produce artifacts in the reconstruction. Considering 
those problems, the presence of the mound in the reconstruc- 
tion was encouraging. However, the results demonstrated the 
need to obtain data from a site that would allow more com- 
plete coverage. 



'Data and tomograms are available upon request from D. R. Tweeton, Twin 
Cities Research Center, Bureau of Mines, Minneapolis, MN. 



80 



FIELD TESTS IN LIMESTONE QUARRY 
Experimental Method 

Because an adequate data set was not obtained from the 
Bureau's on-site test facility, Bureau researchers gathered a 
more complete set of travel times at the Basic Materials, Inc., 
limestone quarry near Waterloo, IA. The test site, shown in 
figure 6, is mostly competent limestone, with some fractured 
zones. Inspection of the nearby pit wall allowed the Bureau to 
select borehole locations between major vertical fractures. 

The source borehole, near the bush on the left side of 
figure 6, was 18.2 m from the receiver borehole, by the seated 
person near the right of the figure. The water injection 
borehole was between them, 6.1 m from the receiver borehole. 
All three boreholes were 18 m deep and 8.9 cm in diameter. 
The water levels before injection were 16.3, 12.5, and 12.7 m 
in the source, injection, and receiver boreholes, respectively. 
The source and receiver positions were from 2.4 to 12.2 m 
below the surface, all above the water table. There were 17 
source positions and 17 receiver positions, directly across from 
each other. Position intervals were 0.61 m. 

The source, shown in figure 7, was an air gun with a 
diameter of 6.4 cm and a 330-cm 3 chamber run at 10.3 MPa 
(1,500 psi). This source provided adequate seismic energy, 
even in these dry boreholes. The receiver was the wall-locking 
geophone receiver system developed at the Bureau. The fre- 
quency transmitted through the limestone was centered at 200 
Hz. The waves were recorded on a 24-channel seismograph. 
Each waveform was recorded with 959 samples (959 dots on 
the seismograph display) at a sampling frequency of 20,000 
Hz. 

Data were first collected when no water was being in- 
jected. After the preinjection tomographic data set was col- 
lected, water was injected into the middle borehole to raise its 
water level to 5.2 m below the surface. The water was main- 
tained at that level by injecting about 2 L/h. Such a low flow 
rate indicates the low permeability of the rock. The water level 
in the other wells did not rise. The lack of a rise in the receiver 
borehole water level did not mean that the limestone near that 
borehole did not become saturated, only that the flow rate was 
not large enough to cause the water level to rise. After the 
water level in the middle borehole had stabilized overnight, a 
second set of tomographic data was collected using the same 
source and receiver positions as for the preinjection set. 

The precision (measurement error) in the data 7 was 0.05 
ms, based on the sampling frequency. The accuracy was 
estimated to be 0.1 ms, based on repeatability. 

Analysis and Results 

The data were analyzed with a curved-ray tomographic 
program. The ray-tracing portion of the program, RAYPS, 
was written by Michael Weber at the University of Frankfurt 



(currently at Seismologisches Zentralobservatorium, 
Grafenberg, Federal Republic of Germany). 8 Chris Calnan of 
the University of Utah added algorithms for calculating the 
travel times for individual receivers. The Bureau added 
algorithms for tomographically calculating the distribution of 
seismic velocities that best fit the travel times. 

The curved-ray tomographic program adjusts the seismic 
velocities at nodes and then calculates the derivatives of veloci- 
ty in triangles with those nodes as corners. The derivatives of 
seismic velocities are used for calculating the bending that oc- 
curs in each triangle. A continuous velocity at the triangle 
boundaries was assumed. There were 16 nodes horizontally 
and 21 nodes vertically. The positions of source and receiver 
were centered between nodes at the edge of the grid. The node 
region was extended below the positions of the source and 
receiver because the paths tended to curve downward toward 
the high-velocity zones. In the bottom row of the tomograms, 
nodes for which there were no nearby paths were left un- 
colored. The only constraint applied in the analyses was to set 
the upper velocity limit to 4.2 km/s. Without that constraint, 
the velocity at some nodes became unreasonably high. 

Five iterations were required to obtain the reconstructions 
shown in figures 8 and 9. There was little qualitative change 
from iteration 4 to iteration 5, although average velocities rose 
slightly as paths were bent more to higher velocity zones. Ad- 
ditional work will be required to improve the stability of the 
curved-ray tomographic program. Ideally, the program should 
be able to run for an unlimited number of iterations without 
departing from a solution. However, when more than 10 itera- 
tions were allowed, the program began to give unreasonably 
high velocities when it was not constrained. 

The reconstruction using preinjection data (fig. 8) showed 
a high-velocity zone sloping upward from near the bottom on 
the source side to the receiver side, with a low-velocity zone 
below the high-velocity zone on the receiver side at a depth of 
10.4 to 11.6 m. The indicated presence of a low-velocity zone, 
implying fractures, is consistent with the tendency of the 
geophone receiver to become stuck at 10 to 1 1 m depth. Unfor- 
tunately, there were no cores or driller's logs for these 
boreholes. 

The reconstruction using data taken during injection is 
shown in figure 9. To facilitate comparing velocities before 
and during injection, figure 10 displays the velocities during in- 
jection minus the velocities before injection. The mound in the 
water level is where it should be. The former low-velocity zone 
near the bottom on the receiver side acquired a higher velocity. 
This change is consistent with the flow of water from the injec- 
tion well into the fractured zone, increasing the seismic veloci- 
ty. An artifact appeared near the top of the grid, where the 
calculated velocity increased, although no water was injected 
there. Also, there were some nodes where the calculated veloci- 
ty decreased. That decrease is probably an artifact of the 



"Data are available upon request from D. R. Tweeton, Twin Cities Research 
Center, Bureau of Mines, Minneapolis, MN. 



•The ray tracing was based on Weber's 1986 Ph.D. thesis, "Die Gauss-Beam 
Methode zur Berechnung Theoretischer Seismogramme in Absorbierenden In- 
homogenen Medien: Test and Anwendung" ("The Gaussian Beam Method for 
Calculating Theoretical Seismograms in Absorbing Inhomogeneous Media: In- 
vestigation and Application"). 




J ■-**■ 






^ i^fii-V 



ST^-1 






33ET***- "^^"Sfc.-:*-^ 



, ,«-^'^? 



BJj r-H--; J. ; .- r h r'"l 



"J. 






'.^ — i , j^~' y - *-. 



AH ^s 












Figure 6.— Limestone quarry field test site. 



81 




Figure 7.— Air gun seismic source. 



82 



reconstruction. Thus, modification of the program to allow 
the application of appropriate constraints will be required. 
The use of sonic borehole logs to determine the seismic 
velocities at the sides of the ray-path region could help to 
reduce artifacts. 

The average velocity was 2.92 km/s before injection and 
3.24 km/s during injection, a 10-pct difference. That change is 
consistent with previous laboratory experiments. It was less 
than the decrease observed in the Bureau site, probably 
because the quarry rock was less fractured. The travel times 
decreased by an average of 5 pet. The reason that average 
velocities increased by a greater percentage than travel times 



decreased is that paths became longer as they bent. Paths bent 
more during injection because of higher velocity contrasts. 

The results were encouraging. The mound was located in 
the right position, and most of the pattern of changing 
velocities was consistent with what was known about the site. 
The data from this experiment and from others demonstrate 
that the seismic velocity does increase when dry rock becomes 
wet under field conditions. Extreme drying or saturating, as in 
the laboratory, was not necessary to observe the effect, and the 
increase in velocity was large enough to measure with commer- 
cial seismic equipment. 



Source 
positions 



Injection 
borehole 




Receiver 
positions 



2.4 m 



12.2 m 



KEY 



1.6 to 2.0 km s 


2.0 to 2.3 km s 


2.3 to 2.6 km s 


2.6 to 2.9 km/s 


2.9 to 3.2 km/s 


3.2 to 3.5 km s 


3.5 to 3.8 km s 


3.8 to 4.2 km/s 



Figure 8.— Seismic velocities before water injection. 



Source 
positions 



Injection 
borehole 




Receiver 
positions 



2.4 m 






12.2 m 



KEY 

1.6 to 2.0 km/s 
2.0 to 2.3 km/s 
2.3 to 2.6 km/s 
2.6 to 2.9 km/s 
2.9 to 3.2 km/s 
3.2 to 3.5 km/s 
3.5 to 3.8 km/s 
3.8 to 4.2 km/s 



Figure 9.— Seismic velocities during water injection. 



83 



Source 
positions 



Injection 
borehole 




Receiver 
positions 



2.4 m 



12.2 m 



KEY 



1.4 to 


-0.8 km/s 


0.8 to 


-0.2 km/s 


0.2 to 


0.2 km/s 


0.2 to 


0.5 km/s 


0.5 to 


0.8 km/s 


0.8 to 


1.2 km/s 


1.2 to 


1.6 km/s 


1.6 to 


2.0 km/s 



Figure 10.— Seismic velocities during injection minus velocities before injection. 



ELECTRICAL AND ELECTROMAGNETIC METHODS 



The seismic method is not expected to be able to 
distinguish between leach solution and ground water. 
Therefore, leach solution injected below the water table will 
not be located by seismic methods. A method that responds to 
a different physical property, such as electrical conductivity, is 
necessary. The conductivity of acidic solutions will usually be 
higher than the conductivity of the surrounding ground water, 
thus providing a good target for electrical and electromagnetic 
methods. Electrical and electromagnetic methods under con- 
sideration include magnetotellurics (MT), galvanic resistivity, 
ground-penetrating radar (GPR), and frequency- and time- 
domain electromagnetics (FEM and TEM). All of these 
methods have been used in locating and monitoring con- 
taminated ground water, chemical spills, and brine plumes and 
are believed to be applicable to leach solution detection as 
well. These methods are discussed in many references, such as 
Telford (19) and the recent comprehensive publication by the 
Society of Exploration Geophysicists (20). 9 

One of the most formidable problems associated with the 
detection of leach solution is the great depth at which in situ 
mining can occur. Some porphyry copper deposits in the desert 
southwest are over 600 m deep, and many geophysical systems 
cannot penetrate that far. In addition, complications arise 
from the presence of conductive overburden, such as saturated 
clays, which greatly attenuate electrical signals. Thick conduc- 
tive overburden blankets various parts of the desert southwest 
where in situ copper mining is being considered. Therefore, 
geophysical systems having deep-penetrating capabilities are 
desirable for this application. 

CSAMT is a modification of earlier MT and audio- 
magnetotelluric (AMT) methods. MT and AMT are natural 
source systems relying on electrical energy provided by 
thunderstorms and related natural activity. Although these 
natural source systems have great depth-penetrating 
capabilities, they suffer from high noise levels caused by un- 



'A second volume, to be published by the Society of Exploration Geologists, 
will give a method-by-method treatment of principal electromagnetic techniques. 



predictable electric fields. Consequently, natural source 
methods would not have the resolution necessary to detect a 
small, deeply buried leach zone. 

CSAMT, on the other hand, uses an artificial or con- 
trolled current source, a transmitter with a long grounded 
dipole or ungrounded loop. Current may be injected at any of 
several frequencies, depending on the depth desired. This 
eliminates the dependence on unpredictable electric fields. 
Although CSAMT does not have the penetration capabilities 
of its natural source counterparts, it does give more accurate 
information. 

Like any electrical or electromagnetic method, CSAMT 
suffers from noise introduced by geologic inhomogeneities, 
conductive overburden, and multiple or nonuniform targets. 
A leach zone, whose conductivity, shape, size, and distribution 
may vary considerably within the ore deposit, may be difficult 
to recognize in one area and easy to recognize in another. Use 
of several loop or dipole sizes and frequencies, as well as use of 
other methods, may be necessary to delineate subsurface solu- 
tions. Further studies will be necessary to determine the 
feasibility of CSAMT for this application, despite its earlier 
failure in the detection of carbonate leach solution (/). 

Galvanic resistivity, or simple resistivity, is a direct- 
current system in which a voltage is measured as the result of 
an applied current. This voltage, which is a function of the 
subsurface resistivity, can be used to map changes in lithology, 
structure, and soil or ground water conductivity. Resistivity 
electrode arrays can be configured in several different ways, 
depending on the survey objective, and can be used in both 
surface and borehole modes. 

Conventional resistivity methods, which depend on a 
point source (single electrode) of current, have limited depth 
of penetration. A modified method, known as focused 
resistivity, holds more promise, since its depth of penetration 
is much greater. 

Focused resistivity relies on three equidistant collinear 
point sources, rather than a single source. Current flowing 
from the outside sources tend to constrain, or focus, the center 



84 



current source. This "focused current" is directed vertically 
downward and propagates a great distance before diffusing, 
giving it deep-penetrating capabilities. This system has been 
used successfully by Southwest Research Institute for detecting 
subsurface cavities or voids 10 and could be applied to deep 
conductive bodies, such as leachate plumes, as well. Focused 
resistivity techniques may also be used in a surface or borehole 
configuration. 

Theoretically, the borehole configuration would be the 
most advantageous, since operation takes place closer to the 
target, but in practice, the borehole configuration is less 
desirable. Cased holes and conductive leachate fluids in the 
holes severely reduce the usefulness of borehole techniques. 
Prior to surveying under such conditions, the casing may have 
to be pulled out or, more likely, the cased portion left un- 
surveyed and fluids pumped out of the hole. Some research 
agencies are investigating three-dimensional resistivity 
measurements using sources that may penetrate through cas- 
ings. 

GPR operates by transmitting a high-frequency pulse into 
the surrounding medium. If this pulse impinges upon an elec- 
trical conductor, such as a leach solution plume, part of the 
pulse will be reflected back to the receiver. In a surface con- 
figuration, GPR can be used to map the lateral extent of the 
plume. In a borehole configuration, a radial picture of media 
surrounding the borehole is obtained. 

Unfortunately, GPR, like all high-frequency systems, suf- 
fers from wave attenuation when used in conductive ground. 
Wave attenuation reduces signal amplitude to such an extent 
that deep exploration with such systems becomes impossible. 
One way to reduce the problem is to decrease the frequency, 
but this tends to accentuate dispersion and distortion effects. 
In addition, power consumption is higher and resolution 
becomes poorer. GPR may be useful in shallow or short-range 
detection and monitoring applications. 

Electromagnetic methods use an alternating current in a 
coil to produce a primary magnetic field. Secondary fields are 



generated when eddy currents swirl inside a subsurface con- 
ductor. These secondary fields produce voltages in receiving 
coils, which are then recorded by the operator. Measurements 
can be made in either time or frequency domains. TEM 
measurements are made by shutting off the primary field, then 
recording the voltage as a function of time. FEM 
measurements are made while the primary field is on. The 
secondary field is then measured as a function of frequency. 
The receiver may be on the surface or in a borehole. 

TEM is better suited for deep detection of leach solution 
plumes than is FEM, especially if conductive overburden 
blankets the area. Conductive overburden tends to diminish 
the signals going to and coming from the target. To measure 
the small signals generated by a conductor under a thick con- 
ductive overburden, it is desirable to increase the signal-to- 
noise ratio. This may be accomplished in several ways. First, 
TEM measures the secondary field in the absence of the 
primary field, thus eliminating the need for exact transmitter- 
receiver-coil separation and orientation. Exact coil separation 
is necessary in FEM surveys to compensate for the primary 
field strength, allowing the much weaker secondary field to be 
measured. Second, TEM measurements facilitate stacking, a 
process by which a measurement is repeated many times in a 
short period of time, which helps to reduce random back- 
ground noise. Finally, TEM systems employ large transmitter 
coils, sometimes several hundred meters across, resulting in 
enormous magnetic dipole moments and hence a large signal. 
Large dipole moments become extremely important when in- 
vestigating conductive bodies buried under thick conductive 
cover. 

Computer simulations are being conducted to investigate 
whether this method has sufficient sensitivity. Preliminary 
calculations are encouraging, and Bureau researchers believe 
that TEM is the most amenable system for locating conductive 
leach solution plumes. After computer simulations are com- 
pleted, the most promising methods will be selected for ex- 
perimental evaluation. 



SUMMARY 



Bureau researchers believe that certain geophysical tech- 
niques are promising as an aid to detecting probable flow 
paths and monitoring leach solution during in situ mining. The 
most promising techniques are seismic cross-hole tomography 
and electromagnetic induction. Seismic cross-hole tomography 
has the potential for being a practical procedure because of the 
relatively low cost of the equipment and the large penetration 



;0 Vv'ork done under Bureau contract H0245005. 



distance. The Bureau has developed components of a seismic 
cross-hole tomography system. Analyses of experimental data 
indicate that the method is promising, but more research will 
be required to minimize mathematical artifacts unrelated to 
the presence of leach solutions. Electromagnetic induction is 
promising in principle because of its ability to detect a good 
conductor in the presence of poor conductors. GPR may be 
useful for detecting fracture zones and leach solutions near the 
borehole, but its limited penetration range will prevent it from 
being used for monitoring. 



85 



REFERENCES 



1. Kehrman, R. F. Detection of Lixiviant Excursions With 
Geophysical Resistance Measurements During In Situ Uranium 
Leaching (contract J01 88080, Westinghouse Electric Corp.)- BuMines 
OFR 5-81, 1979, 157 pp.; NTIS PB 81-171324. 

2. Ramirez, A. L. Recent Experiments Using Geophysical 
Tomography in Fractured Granite. Proc. IEEE, v. 74, No. 2, 1986, 
pp. 347-352. 

3. Dines, K. A., and R. J. Lytle. Computerized Geophysical 
Tomography. Proc. IEEE, v. 67, No. 7, 1979, pp. 1065-1073. 

4. Chiu, S. K. L., E. R. Kanasewich, and S. Phadke. Three- 
Dimensional Determination of Structure and Velocity by Seismic 
Tomography. Geophysics, v. 51, No. 8, 1986, pp. 1559-1571. 

5. Bishop, T. N., K. P. Bube, R. T. Cutler, R. T. Langan, P. L. 
Love, J. R. Resnick, R. T. Shuey, D. A. Spindler, and H. W. Wyld. 
Tomographic Determination of Velocity and Depth in Laterally Vary- 
ing Media. Geophysics, v. 50, No. 6, 1985, pp. 903-923. 

6. Albright, J. N., P. A. Johnson, W. S. Phillips, C. R. Bradley, 
and J. T. Rutledge. The Crosswell Acoustic Surveying Project. Los 
Alamos Natl. Lab. Rep. LA-11157-MS, Mar. 1988, 121 pp. 

7. Gustavsson, M., S. Ivansson, and J. Pihl. Seismic Borehole 
Tomography — Measurement System and Field Studies. Proc. IEEE, 
v. 74, No. 2, 1986, pp. 339-346. 

8. Peterson, J. E., B. N. P. Paulson, and T. V. McEvilly. Applica- 
tions of Algebraic Reconstruction Techniques to Crosshole Seismic 
Data. Geophysics, v. 50, No. 10, 1985, pp. 1566-1580. 

9. Peterson, J. E., Jr. The Application of Algebraic Reconstruc- 
tion Techniques to Geophysical Problems. Ph.D. Thesis, Univ. CA, 
Berkeley, CA, LBL-21498, 1986, 188 pp. 

10. Foss, M. M., and R. J. Leckenby. Coal Mine Hazard Detection 
Using In-Seam Ground-Penetrating-Radar Transillumination. 
BuMines RI 9062, 1987, 27 pp. 



11. Werniuk, J. Tomographic Vision Into Solid Rock. Can. Min. 
J., v. 109, No. 7, 1988, pp. 24-25. 

12. Worthington, M. H. An Introduction to Geophysical 
Tomography. First Break, v. 2, pt. 11, Nov. 1984, pp. 20-26. 

13. Thill, R. E., and J. A. Jessop. Effects of Water Saturation on 
Acoustic Wave Velocity. Soc. Min. Eng. AIME preprint 86-148, 1986, 
13 pp. 

14. Ivansson, S. Seismic Borehole Tomography — Theory and 
Computational Methods. Proc. IEEE, v. 74, No. 2, 1986, pp. 
328-338. 

15. Wattrus, N. J. Two-Dimensional Velocity Anomaly 
Reconstruction by Seismic Tomography. Ph.D. Thesis, Univ. MN, 
Minneapolis, MN, 1984, 245 pp. 

16. Tweeton, D. R. A Tomographic Computer Program With Con- 
straints To Improve Reconstructions for Monitoring In Situ Mining 
Leachate. BuMines RI 9159, 1988, 70 pp. 

17. Cumerlato, C. L., V. J. Stachura, and D. R. Tweeton. Applica- 
tion of Refraction Tomography To Map the Extent of Blast-Induced 
Fracturing. Paper in Key Questions in Rock Mechanics: Proceedings 
of 29th U.S. Symposium (Minneapolis, MN, June 13-15, 1988). A. A. 
Balkema, 1988, pp. 691-698. 

18. Sjogren, B. Seismic Classification of Rock Mass Qualities. 
Geophys. Prospect., v. 27, 1979, pp. 409-442. 

19. Telford, W. M., L. P. Geldart, R. E. Sheriff, and D. A. Keys. 
Applied Geophysics. Cambridge Univ. Press, 1976, 860 pp. 

20. Nabighian, M. N. (ed.). Electromagnetic Methods in Applied 
Geophysics, Volume I, Theory, v. 3 in Series Investigations in 
Geophysics. Soc. Exploration Geophysicists, 1988, 513 pp. 



86 



ENHANCED WELL-DRILLING PERFORMANCE WITH 
CHEMICAL DRILLING-FLUID ADDITIVES 



By Patrick A. Tuzinski, 1 John E. Pahlman, 2 Pamela J. Watson, 3 and William H. Engelmann 4 



ABSTRACT 

The U.S. Bureau of Mines has investigated the use of chemical additives to enhance 
drilling performance, for which conventional hard-rock drilling and in situ borehole 
development are target applications. Laboratory diamond core drilling tests were performed 
on Sioux Quartzite, Westerly Granite, Minnesota taconite, and Tennessee marble, using 
solutions of cationic additives, nonionic polymer, and acid-base buffered solutions as drill- 
ing fluids. Penetration improvements ranged from 88 to over 650 pet and bit life im- 
provements from 56 to over 400 pet with chemical additive solution concentrations that pro- 
duced a zero surface charge (ZSC) on the rock. The ZSC condition was found to be the most 
important factor in improving drilling performance. The nonionic polymer was found to be 
the best additive in the laboratory drilling tests and the most appropriate for field use 
because the ZSC condition could be obtained over a wide range of solution concentrations 
(10 to 125 ppm). Field validation tests of rotary tricone drilling with polymer on Minnesota 
taconite increased penetration rate by about 69 pet and bit life by about 26 pet. Application 
of this enhanced drilling phenomenon to well drilling for in situ mining should be pursued 
because of the potential for time and cost savings. 



INTRODUCTION 



As part of an overall program to increase minerals extrac- 
tion efficiency, the Bureau of Mines has investigated the use of 
chemical additives in drilling fluids to enhance drilling perfor- 
mance in hard rock. Watson (7) 5 surveyed and summarized 60 
yr of literature-reported results of using chemical additives in 
rock fragmentation processes. In that 60-yr period, some 
researchers reported no improvements when drilling with ad- 
ditives in the drilling fluid while others reported significant 
benefits, especially when drilling with additive solutions that 
create an electrochemical point of zero charge (PZC) or zero 
surface charge (ZSC) on the surface of the rock being drilled. 
Some noteworthy successes include Rehbinder's work (2), in 
which drilling efficiency increases of up to 60 pet were 
reported; Shepherd's work (3), in which slightly lower in- 
creases in drilling efficiencies were obtained; and Westwood's 



'Research geochemist. 
Supervisory physical scientist. 
'Mining engineer. 
■■Research chemist. 

Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN. 
'Italic numbers in parentheses refer to items in the list of references at the end 
of this paper. 



work (4), which indicated that drilling efficiency could be im- 
proved well over 100 pet. The survey showed that, while the 
use of chemical additives in the drilling fluid was sometimes 
beneficial, little scientific reasoning was provided in the 
literature to explain either the presence or absence of beneficial 
effects when these chemical additives were used. 

Intrigued by these reports, the Bureau designed and con- 
ducted laboratory drilling studies to sort out the disparities of 
earlier research and to establish and understand the necessary 
boundary conditions for drilling performance improvement 
under ZSC conditions. This paper describes the laboratory 
research conducted to determine the boundary conditions of 
the enhanced ZSC-controlled drilling phenomenon and to 
determine the applicability of the phenomenon with respect to 
(1) rock type, (2) bit type, and (3) surface charge (zeta poten- 
tial) modifier. Also included are results from several initial 
field validation tests of ZSC-controlled drilling, which suggest 
that drilling with the recommended additive could produce 
significant drilling performance improvements. One potential 
application for the use of additive-assisted drilling is in the 
development of in situ mining deposits, for which considerable 
time and cost savings could be realized. 



87 



ROCK MATERIALS USED IN TESTS 



All rock samples for laboratory drilling tests were wire 
sawed into 15-cm cubes. Rock fragments of the same samples 
were ground to minus 149 ^m for zeta potential measurements 
and chemical analyses. Previous Bureau reports have given 
detailed descriptions of Sioux Quartzite and Westerly Granite, 
including their physical properties (5), and Minnesota taconite 
and Tennessee marble (6). Briefly, Sioux Quartzite is a 
homogeneous, fine-grained, metamorphosed sandstone that 
has a relatively fracture-free structure, is comprised mainly of 
quartz grains, and averages 98.5 pet silica. Samples were ob- 
tained from a quarry in southwestern Minnesota. Westerly 
Granite is a light-gray, fine-grained, equigranular 
granodiorite, containing about 65 pet feldspars, 25 pet quartz, 
9 pet micas, and about 1 to 2 pet accessory minerals. The 
Westerly Granite used in this research was obtained as a single 
1,500-lb slab from an active quarry in Bradford, RI. Min- 



nesota taconite is a dark green to gray, fine-grained, 
metasedimentary rock consisting of chert, magnetite, 
hematite, siderite, and the following silicates: minnesotaite, 
greenalite, stilpnomelane, and amphibole. The taconite has 
mildly undulating black bands of magnetite that occur in ir- 
regular layers, which account for the overall 27 to 30 pet iron 
content. The tactonite was provided by Erie Mining Co., 
located near Hoyt Lakes, MN. Tennessee marble (quarry trade 
name) used in this investigation came from an active mine in 
the Holston Limestone formation in the Great Valley of east 
Tennessee (7). The formation is an essentially unmetamor- 
phosed, coarsely crystalline limestone of Middle Ordovician 
age. The formation shows a range of colors from light gray to 
pinks and red to dark brown. Chemically, the rock is about 
97.4 pet calcium carbonate, with trace amounts of other alkali 
minerals. 



WATERS USED IN TESTS 



Four waters were used in the drilling tests: distilled, 
deionized water (DDIW), tap water, mine pond water, and 
mine well water. Chemical analyses of these waters are given in 
a previous report (6). 

DDIW was prepared by distilling Minneapolis tap water 
in a high-capacity 200-L still and then passing it through stand- 
ard and ultrapure ion-exchange cartridges. The pH of the 
DDIW was in the range 5.3 to 6.0, and the conductivity 
measured from 0.3 to 0.5 ^mho/cm. 

The Minneapolis tap water used in these tests had an 
average pH value of 7.3 to 7.7, and the conductivity was about 
200 ^mho/cm. 



Both the mine pond water and mine well water were ob- 
tained from Erie Mining Co. in northern Minnesota. The mine 
pond water, used as a source of water at Erie Mining during 
the spring, summer, and fall, is obtained from a pond at the 
bottom of one of the company's open pits. The mine well 
water, used as a source of water during the winter, is pumped 
from a well on the property. The pH of the pond water ranged 
from 7.0 to 7.5, while the pH of the well water was 7.9 to 8.0. 
Both had conductivities from 500 to 700 ^mho/cm. 



SURFACE CHARGE CONSIDERATIONS 



The surface charge of most rocks in water around neutral 
pH is negative. If sufficient amounts of surface charge 
modifiers are added to the drilling fluid, such as inorganic 
salts, cationic surfactants, cationic polymers, and a special 
nonionic polymer, the surface charge can be neutralized, 



resulting in the ZSC condition. The zeta potential gives a 
measure of rock surface charge. Relationships between zeta 
potential, double-layer theory, and surface charge have been 
discussed in a previous report (5). 



ZETA POTENTIAL MEASUREMENTS AND ZSC CONCENTRATIONS 



The procedure used for measuring zeta potentials and 
determining the ZSC concentration is described in detail 
elsewhere (5). Using a commercial zeta reader, zeta potentials 
are determined for the rock particles in water (DDIW, tap, or 
mine water) while concentrations of additive solutions are in- 
creased. 

The zeta reader operates on the principle of elec- 
trophoretic mobility, whereby the speed of a particle in an 
electric field is proportional to its surface charge. The ap- 
paratus employs a video display to monitor particle movement 
in the electric field. When the speed of a moving grid line on 
the video display is matched to that of the particle, the zeta 
potential of that particle is shown on a digital readout. 



For the drilling experiments tested, zeta potentials were 
determined for each rock type in water alone and in water with 
a series of concentrations of each additive. This is done to 
generate the range of negative and positive zeta potential 
values needed to graphically determine the concentration at 
which the zeta potential or surface charge of the rock particles 
is zero. It is important to determine each water-rock combina- 
tion because ZSC concentrations were found to be substantial- 
ly different for each rock tested and depend both on the 
chemical additive and type of water used. 



88 



DRILLING SYSTEM 



The drilling apparatus employed in this investigation is 
shown in figure 1. A detailed description of the mechanical 
and electronic components of the drilling apparatus and the 
drilling procedure are given in a previous report (5). Briefly, 
drilling tests were performed on 15-cm rock cubes using a 
1.12-kW drill press, fitted with a water swivel, and either a 
16-mm-OD (10-mm ID) diamond-impregnated coring bit or a 
16-mm tungsten carbide water-cooled spade bit. 

The diamond-impregnated coring bits were rotated at a 
speed of 100 rpm under a thrust of 150 kg; the spade bits were 
also rotated at a speed of 100 rpm but under a lower thrust of 
60 kg. Drilling fluid was flushed through either drill at a rate of 
150 mL/min. Drilling was done perpendicular to the bedding 
plane of rocks where applicable. 

The coring bit matrix was 100 pet cobalt (powder) sintered 
to a Rockwell C hardness of 18 to 21. The diamonds in the 
matrix were quoted by the manufacturer to be minus 425 fim 
plus 300 /xm, although the size range of the diamonds was ac- 
tually minus 1,000 plus 250 ^m. The tungsten carbide spade 
bits were used only to drill the Tennessee marble. These bits 
featured replaceable, resharpenable blades with two water- 
ways. Assembled, they resemble a machinist's straight-fluted- 
type drill bit of 16 mm diameter. The tip had a 125° included 
angle, a 125° chisel angle, and a 5° to 8° negative axial rake 
angle. 

The diamond coring drill bits were sharpened by briefly 
drilling into a superduty fireclay brick (53 pet silica, 42 pet 
alumina) to produce a sharpness level corresponding to an ini- 
tial average penetration rate for one hole of 4.5 mm/min in the 
test rock sample. The tungsten carbide blades were factory 
sharpened to a uniform degree; therefore, they were not 
resharpened in the laboratory. Drilling continued for a se- 
quence of several holes until the average penetration rate for a 
hole had dropped to 2.0 mm/min in the test rock sample. The 
diamond coring drill bits were resharpened before another test 
began; the tungsten carbide blades were simply changed for 
the next text. 

Data on total penetration and elapsed time of drilling in 
progressing from the sharp-bit state of 4.5 mm/min to the 
dull-bit state of 2.0 mm/min were recorded, added, and then 
used for test comparisons. The enhanced penetration perform- 
ance of the additive, as compared with the appropriate 
baseline water, was calculated by 



The bit life effect of the additive, as compared with the 
baseline water, was calculated by 



E p = [C p - W p )/W p ] • 100, 



(1) 



where E 



and 



- penetration effect for a test, pet, 

C p = total penetration for drilling with a given ad- 
ditive, mm, 

W p = average total penetration for drilling with water 
alone, mm. 



E, = [(C, - W,)/W ( ] • 100, 



(2) 



where E t 

C, 
and W, 



bit life effect for a test, pet, 

total time for drilling with a given additive, min, 

average total time for drilling with water alone, 

min. 



The results of penetration and bit life effects for additive con- 
centrations compared with their respective baseline water tests 
are described in the following section. 




Figure 1.— Drilling apparatus. 



LABORATORY DRILLING TEST RESULTS AND DISCUSSION 

Results of laboratory drilling tests are presented in table 1 and figures 2 through 1 1 . 



Table 1.— Drilling results for all additives, percent improvement 



89 



Type of water, additive, 
and additive concentration 



Penetration 



Bit life 



Type of water, additive, 
and additive concentration 



Penetration 



Bit life 



SIOUX QUARTZITE 



SIOUX QUARTZITE— Continued 



DDIW 
1.0 
1.0 

5.0 

1 6.0 
7.0 
8.0 
1.0 
3.0 
5.0 
7.0 
1.0 
5.0 
1.0 
DDIW 
1.0 
1.0 
1.0 
5.0 
7.0 
9.0 

1 1.1 
2.0 
3.0 
4.0 
5.0 
7.0 
1.0 
DDIW 
9.0 
1.0 

'2.0 
3.0 
5.0 
DDIW 
1.0 
5.0 
9.0 

'1.4 
5.0 
1.0 
1.0 
DDIW 
1.0 
3.0 

1 4.3 
5.0 
1.0 



CaCI 2 , mol/L: 



with AICI 3 , mol/L: 

x10" 8 

x10" 7 

x10~ 7 

x10" 7 

x10~ 7 

x10" 7 

x10" 6 

x10" 6 

x10" 6 

x10" 6 

x10" 5 

x10" 5 

x10" 4 

with 

x10" 

x10" 

x10" 

x10~ 

x10" 

x10" 

x10" 

x10~ 

x10" 

x10" 

x10" 

x10" 

x10~ 

with 

x10" 

x10" 

x10" 

x10" 

x10" 

with 

x10" 

x10" 7 

x10" 7 

x10" 6 

x10" 6 

x10" 5 

x10" 4 

with AI(N0 3 ) 3 , mol/L: 

x10" 7 

x10" 7 

x10" 7 

x10" 7 

x10" 6 



NaCI, mol/L: 
-2 



ZrCI 4 , mol/L: 
-7 



- 33.53 
40.12 
98.50 

103.83 
62.87 
61.99 
28.60 
17.21 
52.47 
35.81 
79.64 
60.32 
50.40 

.87 

53.23 

28.32 

87.32 

32.60 

68.12 

96.53 

-14.18 

-7.46 

1.42 

-17.18 

-3.70 

1.56 

16.52 

14.86 

115.03 

-3.50 

2.22 

33.32 
-9.57 
39.54 
96.29 
4.48 
43.37 

- 46.33 

-14.34 
84.46 
98.09 

- 24.32 
11.38 



-33.76 
44.69 
98.71 
85.12 
35.02 
40.96 
23.13 
22.28 
50.31 
34.17 
73.23 
44.36 
54.55 

13.81 
36.01 
24.83 
73.23 

5.30 
52.85 
72.67 

5.74 
-1.49 

5.30 
-6.59 
10.39 
20.58 

5.30 

5.30 

76.35 

-6.59 

15.49 

40.12 
-1.49 

35.87 

67.71 
-8.29 

30.77 
-36.31 

-5.01 
55.16 
63.50 
-16.35 
18.82 



DDIW with DTAB, mol/L: 

3.0x10" 4 

^xlO" 4 

1.5x10" 3 

DDIW with TTAB, mol/L: 

5.0x10" 6 

9.0x10" 6 

1.7x10~ 5 

1.85x10" 5 

2.0x10" 

2.2x10" 

2.8x10" 

5.0x10" 



-5 
-5 

>-5 
-5 



-6 

,-6 



1 7.23x10 -5 

9.0X10" 5 

DDIW with HTAB, mol/L: 

5.0x10 -7 

9.0x10 -7 

1 1.6x10 -6 

3.0x10" 

5.0x10" 
Tap water with PAA, ppm: 

0.10 

0.15 

0.20 

1 0.25 

0.50 

1.0 

Tap water with PEO, ppm: 

1.0 

1 3.0 

7.5 

12.5 

125.0 

Tap water at pH of 3.8 



with AICI3 
7.0x10"' 
1.0x10"' 
1.15x10" 
1.18x10" 
1.6x10" 
1.7x10"' 

1 1.8x10"' 
1.9x10"' 
2.0x10"' 
3.0x10"' 



mol/L: 



- 32.92 
118.18 
111.36 

- 34.79 
-5.82 

-.19 

1.08 

13.31 

-17.47 

-45.20 

-35.11 

87.84 

-6.86 

9.27 
71.67 
87.70 

- 10.22 
-3.94 

- 16.54 

47.99 

209.17 

334.03 

174.83 

26.82 

66.16 
429.65 
348.56 
363.65 
387.60 



45.37 
46.20 
46.27 
80.51 
80.74 
95.82 

113.63 
97.02 
33.06 

- 26.82 



- 27.39 
99.56 

108.41 

-33.11 
-7.83 

-4.75 

-4.31 

-3.14 

-11.64 

- 45.60 
-33.51 

87.02 
22.60 

1.26 

54.57 

55.94 

-10.80 

-3.27 

-15.52 

29.18 

183.14 

187.22 

124.49 

22.02 

45.55 
270.58 
235.69 
235.90 
330.05 



16.07 
49.78 
20.23 
48.97 
66.31 
34.06 
59.59 
76.94 
6.47 
29.30 



See footnotes at end of table. 



90 



Table 1.— Drilling results for all additives, percent improvement— Continued 



Type of water, additive, 
and additive concentration 


Penetration 


Bit life 


Type of water, additive, 
and additive concentration 


Penetration 


Bit life 


WESTERLY GRANITE 


MINNESOTA TACONITE- 


-Continued 




DDIW with AICI3, 
1.0x 10~ 7 . . . 
3. Ox 10~ 7 . . . 


mol/L: 


18.66 

62.05 

133.75 

104.50 


0.83 

67.28 

92.72 

73.92 

109.00 

-8.46 

-22.70 

136.23 

- 29.23 

-22.07 

-52.01 

-16.92 


Mine well water with 
Percol 402, ppm: 
0.40 


105.66 

174.98 

-7.73 

9.90 

31.00 
138.95 
278.63 

19.43 

107.80 
246.76 
647.52 
661.94 


92.30 


5. Ox 10~ 7 . . . 


1 0.64 


141.40 


6.0 x 10" 7 


1.0 


-3.48 


1 7 Ox 10~ 7 . . . 




154.66 


6.4 


22.34 


I.Ox 10" 6 . . . 
1 5x 10" 6 . . . 




-17.45 

-16.81 

164.92 


Buffered pH solutions, pH units: 

7.0 


54.70 


'2.0x 10~ 6 . . . 


6.0 


129.15 


3.0x 10~ 6 . . . 




- 36.42 

-19.79 

66.72 

-16.31 


1 5.5 


218.14 


5.0x 10" 6 . . . 


3.5 


49.29 


7.0x10" 6 . . . 
1 Ox 10" 5 


Mine pond water with PEO, ppm: 

3.0 


130.66 




7.5 






MINNESOTA TACONITE 




220.14 




1 12 5 


421.28 
603.67 


DDIW with AICI3, 


mol/L: 




42.68 

28.25 

71.15 

104.46 

104.07 

6.14 

37.50 


125.0 








9.0x10" 7 . .. 




53.93 


TENNESSEE MARBLE 


1 Ox 10" 6 . . . 




70.20 


DDIW with AICI3, mol/L: 

4.0x10" 7 


15.94 
25.07 
50.09 
84.25 




1 1.2x10~ 6 . . 




143.45 


39.65 


1.4x10 -6 ... 
1.7x10" 6 . . . 
2.0x 10" 6 . . . 




135.55 

22.48 

34.17 


1.6X10 -6 

1 1.9x1(T 6 

2.3x10" 6 


50.18 

98.18 

146.09 









' Point of zero charge concentration. 

2 Compared with baseline water of same pH 3.8 level. 



91 



6x10 7 mol/L 




g 125 

a. 100 
| 75 

UJ 

Q- 50 
O 25 
t 
t-25h 

UJ 

uj-50 - 
> 10 4 



Q 
Q 

< 



C, NaCI 



'2xl0~' mol/L 1 



Water baseline 
L 




10 



,-2 



10" 



I0" 4 I0" 5 



125 
100 

75 

50 

25 

-25 h 
-50 



- f.AKNOsh, _ 4 . 33x|0 -7 mo|/L 



1 

B, CaCI 2 


1 1 
1 .13 x I0 2 mol/L-*, 






- 


o f\ 




- 


- s' 


^^**^^>*^ o 






1 


/ 

Water baseline^ 

l I 


V o 





A ZrCI 4 



v* 





Water baseline 



_L 



10" 



10 



,-6 



10" 



10" 



ADDITIVE CONCENTRATION, mol/L 
Figure 2.— ZSC control of drilling penetration in Sioux Quartzite with inorganic salts in DDIW. 



92 




Water baseline- 
I 



125 

100 

75 

50 

25 



-25 



> -50 
t 10 

o 

Q 

< 



-4 



10" 

125 
100 

75 

50 

25 

-25 h 
-50 



E, AI(N0 3 ) 3 



10" 



10" 



10" 



1 

C, NaCI 


1 
2 x |o" 


1 
mol/L-»j 

A 


? 


1 


Water basel 

l 


ne^ 

1 


- 



1 
_ B, CaCI 2 


1 1 


- 


- 


1.13x1 0~ 2 mol/L-^j 


- 


- ^^-" 




\'S* 


1 


Water baseline 

1 1 


er o 



10" 



10" 



10 



D, ZrCI 4 



Water baseline 
J I 



I0 U 10 



10 



10" 



10" 




4.33xl0 7 mol/L 



Water baseline- 
_| L 



10 



10" 



10 



,-4 



ADDITIVE CONCENTRATION, mol/L 
Figure 3.— ZSC control of bit life in drilling Sioux Quartzite with inorganic salts in DDIW. 



10 




93 



175 

150 

125 

100 
75 
50 - 
25 - 



o 

CL 







h -25 

u 

UJ 

u- -50 
u_ 



/4, Penetration 



'7xlO" 7 mol/L 7 



2x|0~ 6 mol/L' 



Water baseline 




> 



Q 
O 
< 



175 

150 - 

125 - 

100 - 

75 

50 - 

25 - 







-25 h 
-50 



B, Bit life 



2*10 mol/L 



7x|0 mol/L 



Water baseline 
l__ 




_L 



10 



10 10"° 10" 

ADDITIVE CONCENTRATION, mol/L 



10 



Figure 4.— ZSC control of drilling penetration and bit life in Westerly Granite with aluminum chloride in DDIW. 



94 



o 

Q. 



o 

UJ 
U_ 
U_ 
UJ 



180 

160 

140 

120 

100 

80 

60 

40 

20 



-20 

-40 

-60 



J. AICI 3 inDD'lW 

l.2x|0" 6 — 
mol/L 




Water baseline- 



.-6 



10 10 w 10 

ADDITIVE CONCENTRATION, mol/L 



300 

280 

260 

240 

220 

200 

180 

160 

140 

120 

100 

80 

60 

40 

20 





■ B, Buffered n< — p h 5.5 
•pHinDDIW 



I 




7 



II 13 



ADDITIVE CONCENTRATION, pH 



< 
or 

h- 

LJ 

Ld 
Q_ 


180 
160 
140 
120 




100 




80 




60 




40 




20 









-20 




-40 

0. 




ADD 



C;Percol403 

in mine 

well 

water 



Water baseline 




0.64 ppm_ 



I 0.3 0.6 1.5 4.0 
ITIVE CONCENTRATION, ppm 



800 
700 
600 
500 
400 
300 
200 
100 




ZSConset- 
between 
-7.5-I2 ppm 



-100 



D, PEO in mine water 



Water baseline 



1.0 2.5 7.0 16.0 40.0 100.0 150.0 
ADDITIVE CONCENTRATION, ppm 



Figure 5.— ZSC control of drilling penetration in Minnesota taconite with aluminum chloride, acid, Percol 402, and PEO in 
various waters. 



95 



o 

Q. 



(J 

UJ 
Ll_ 
U_ 
UJ 

UJ 
Li- 



on 



180 

160 

140 

120 

100 

80 

60 

40 

20 



-20 

-40 

-60 



AAICL in DDIW 



1.2x10 
mol/L 



Water baseline 




^-7 



>-6 



10' I0 V 10 

ADDITIVE CONCENTRATION, mol/L 



180 

160 

140 

120 

100 

80 

80 

40 

20 



-20 

-40 



ADD 



i 1 1 

C, Percol 402 in mine wel L 

water _ -.-„ 

—0.64 ppm 



Water baseline 




.1 0.3 0.6 1.6 4.0 
ITIVE CONCENTRATION, ppm 




I 3 5 7 9 II 13 

ADDITIVE CONCENTRATION, pH 



800 

700 - 

600 - 

500 - 

400 - 

300 - 

200 

100 

0* 

-100 



— i i 1 r 

D, PEO in mine pond water 




ZSC onset between 
7.5- 12 ppm 



Water baseline 
1 i 



1.0 2.5 7.0 16.0 40.0 100.0 150.0 
ADDITIVE CONCENTRATION, ppm 



Figure 6.— ZSC control of bit life in drilling Minnesota taconite with aluminum chloride, acid, Percol 402, and PEO in various 
waters. 



96 



80 



o 

Q. 


160 


H 


140 


O 

UJ 


120 


Li_ 


100 


LU 


80 


o 


60 


h- 


40 


< 


20 


Ul 





-z. 

UJ 


-20 


Q_ 


-40 

1 




180 




160 


o 

Q. 


140 


H 


120 


O 

UJ 


100 


u_ 
u. 


80 


Ul 


60 


UJ 

u. 


40 


Ij 


20 


h- 





CD 


-20 




-40 



L2.3x|0~ 6 mol/L 




Water baseline 



0' 



-7 



10 



r6 



2.3x|0~ 6 mol/L 




Water baseline 



<-i 



n-6 







r5 



0' 10 10 

ADDITIVE CONCENTRATION, mol/L 



Figure 7.— ZSC control of drilling penetration and bit life in Tennessee 
marble with aluminum chloride in DDIW. 



97 



o 

Q. 

O 

LlI 

u_ 
u_ 

UJ 



r- 

< 
or 

t- 

UJ 

UJ 
Q_ 



180 

160 

140 

120 

100 

80 

60 

40 

20 



-20 h 

-40 



A, DTAB in DDIW 



9.8x10 

mol/L 







-4 




Water baseline- 



-#,TTAB inDDIW 







-3 



o" 2 I0" 6 



1.6 10 mol/L ^ 



180 

160 \C, HTAB in DDIW 

140 

120 

100 

80 

60 

40 

20 

-20 r 
-40 



Water baseline 



-7 



.-6 



7.2x10 mol/L 




10 



-5 




10 10 10 

ADDITIVE CONCENTRATION, mol/L 

Figure 8.— ZSC control of drilling penetration in Sioux Quartzite with cationic surfactants in DDIW. 







.-4 



98 



o 

Q. 



O 

LJ 
L_ 
U_ 
UJ 



CD 



180 
160 
140 
120 
100 

80 

60 

40 

20 

-20 h 
-40 



A, DTAB in DDIW 



9.8*10 
mol/L 







-4 




Water baseline 



B, TTAB in DDIW 



7.2x10 mol/L 



Water baseline 




10 



-3 



10 



-2 



10 



-6 



10 



-5 



180 

160 |- C, HTAB in DDIW 

140 

120 

100 

80 

60 

40 

20 

-20 \ 
-40 



l.6x|0 mol/L 



Water baseline 




.-7 



^-6 



10 10" 10 

ADDITIVE CONCENTRATION, mol/L 



v-5 



10 



Figure 9.— ZSC control of bit life in drilling Sioux Quartzite with cationic surfactants in DDIW. 



99 



o 

Q. 



o 

Ld 
Q_ 
U_ 
UJ 



O 

< 
or 

\- 

UJ 

-z. 

UJ 
Q_ 



.8x|0 4 mol/L- 



180 

160 |- A AICI 3 in pH-adjusted tap water . 

140 
120 
100 

80 

60 

40 

20 

-20 h 
-40 



Water baseline 




.8, PAA in tap water 
—0.25 ppm 



-C, PEO in tap water 

a 



10 10 10 

ADDITIVE CONCENTRATION, mol/L 



450 

400 

350 

300 

250 

200 

150 

100 

50 





v-3 




0.3 0.6 1.6 4.0 
TIVE CONCENTRATION, ppm 



-50 



ZSC onset between 
3-7.5 ppm 



Water baseline 



1.0 2.5 7.0 16.0 40.0 100.0 150.0 
ADDITIVE CONCENTRATION, ppm 



Figure 10.— ZSC control of drilling penetration in Sioux Quartzite with aluminum chloride, PAA, and PEO in various waters. 



100 



o 
Q. 



O 

L±J 



UJ 
Ld 



0D 



180 

160 

140 

120 

100 

80 

60 

40 

20 



-20 

-40 
I 



_ A, AICI3 in pH-adjusted tap water _ 



1.8X10" mol/L 



>4 



_ Water baseline 



V 



.-5 



.-4 



0~ 10 10 

ADDITIVE CONCENTRATION, mol/L 

450 
400 
350 
300 
250 
200 

150 

100 

50 



-50 



v-3 



450 

400 

350 

300 

250 

200 

150 

100 

50 



-50 
0. 
ADDI 



_B, PAA in tap water 



0.25 ppm 




Water baseline^^ 

1 1 L 



I 0.3 0.6 1.6 4.0 
TIVE CONCENTRATION, ppm 



,C, PEO 


1 
in tap 


1 
water 


1 


1 


- 








□ 















a c 






- / 




^ZSC onset bet 
3-7.5 ppm 


ween 


Water 

1 


basel 


ine— ^ 

1 


1 


1 



1.0 2.5 7.0 16.0 40.0 100.0 
ADDITIVE CONCENTRATION, ppm 



150.0 



Figure 11.— ZSC control of bit life in drilling Sioux Quartzite with aluminum chloride, PAA, and PEO in various waters. 



EFFECT OF ROCK TYPE ON ZSC-CONTROLLED 
DRILLING 

Determination of the rock's response to ZSC-controlled 
drilling involved testing of rocks of high-, medium-, and no- 
silicate content. Sioux Quartzite and Westerly Granite were 
chosen as representative high-silicate rock types, Minnesota 
taconite was chosen as the representative medium-silicate rock 
type, and Tennessee marble (Holston Limestone) was chosen 
as the representative nonsilicate rock type. 



Sioux Quartzite 

Penetration and bit life effects for drilling Sioux Quartzite 
with various concentrations of aluminum chloride (AlCl 3 ) 
solutions, compared with drilling using DDIW, are given in 



table 1 . Drilling with aluminum chloride at the ZSC concentra- 
tion of 6 x 10" 7 mol/L produced a maximum increase in 
penetration of 104 pet and a near maximum in bit life of 85 pet 
(figs. 2A-3A). 



Westerly Granite 

Figure 4 shows plots of penetration and bit life effects for 
drilling Westerly Granite as a function of aluminum chloride 
concentration, while table 1 lists the penetration and bit life ef- 
fects for each aluminum chloride solution concentration 
tested. Maximums in penetration effect of 155 and 165 pet, 
respectively, and in bit life effect of 109 and 136 pet were ob- 
tained when drilling with two concentrations of aluminum 
chloride. These maximums correspond to the ZSC concentra- 
tion for the two main components of the granite: quartz at 



101 



7xl0" 7 mol/L and feldspars at 2xl0~ 6 mol/L aluminum 
chloride. The origin of the third maximum is not understood 
at this time. It was concluded from these results and those for 
Sioux Quartzite that ZSC-controlled drilling is applicable to 
rocks containing high amounts of silicate minerals. 

Minnesota Taconite 

Penetration and bit life effects comparing drilling of Min- 
nesota taconite with various concentrations of aluminum 
chloride to drilling with DDIW are given in table 1. Drill- 
ing with aluminum chloride at the ZSC concentration of 
1.2x 10" 6 mol/L produced simultaneous maximum increases 
in penetration and bit life of 143 and 104 pet (figs. 5A-6A). It 
was concluded from these results that ZSC-controlled drilling 
is also applicable to rocks containing moderate amounts of 
silicate. 

Tennessee Marble 

Penetration and bit life effects comparing drilling of Ten- 
nessee marble with various concentrations of aluminum 
chloride are given in table 1 . The drag, or spade-type, bit was 
used only for drilling into this rock type. Drilling with 
aluminum chloride at 2.3xl0~ 6 mol/L, very near the ZSC 
concentration of 1.93 x 10~ 6 mol/L, resulted in a peak in- 
crease in penetration improvement of 84 pet and a bit life ex- 
tension of 146 pet (fig. 7). 

It is concluded from the drilling results for Sioux Quart- 
zite, Westerly Granite, Minnesota taconite, and Tennessee 
marble that ZSC-controlled drilling should be applicable to all 
rock types, whether they have high-, medium-, or no-silicate 
contents. 



EFFECT OF BIT TYPE ON ZSC-CONTROLLED 
DRILLING 

ZSC-controlled drilling enhancement with diamond- 
impregnated bits has been demonstrated for Sioux Quartzite, 
Westerly Granite, and Minnesota taconite. Drilling of Ten- 
nessee marble was done with a tungsten carbide spade-type bit 
to determine whether ZSC-controlled drilling was applicable 
to other bit types. 

As noted by Westwood (4), the cutting mechanisms of the 
diamond coring and tungsten carbide spade bit types are suffi- 
ciently different. Conventionally, spade bits as described here 
are not used in rock drilling, but rather in the machine-tooling 
trades. However, if a rock is not too hard, it can be drilled 
with a spade bit, which works with a scraping or dragging mo- 
tion. Often the depth of cut for sharp spade bits is greater per 
pass than diamond bit cutting, all other conditions being 
equal, but the bits wear down quickly. 

The demonstration of successful ZSC-controlled drilling 
enhancement for both drag bit types, i.e., diamond drills and 
spade bits, strongly implies the applicability of ZSC-controlled 
drilling to these types of bits. It is furthermore suggested from 
these results that ZSC-controlled drilling should be applicable 
to all drag bit types and possibly to all drill bit types. 

Drilling of Tennessee marble with the diamond- 
impregnated coring bit was also tried; however, no decline in 
drilling rate was observed, even after numerous test holes. This 
meant that any diamond drilling tests using sharp- and dull-bit 
states, as defined by this study, would take far too long to be 
practical in the test program. 



EFFECT OF SURFACE CHARGE MODIFIER ON 
ZSC-CONTROLLED DRILLING 

Inorganic Salts 

Drilling tests on Sioux Quartzite were conducted using 
various concentrations of solutions of the chloride salts of 
aluminum (AlCh), calcium (CaCl 2 ), sodium (NaCl), and zir- 
conium (ZrCLt), as well as aluminum nitrate [A1(N0 3 )3], as 
drilling fluids. The resulting penetration and bit life effects are 
given in table 1 and plotted in figures 2 and 3. These figures 
show that maximum penetration and bit life improvements 
were obtained with ZSC concentrations of each of the five in- 
organic salts tested. Although higher ZSC concentrations were 
required for lower valence cations, the enhanced drilling per- 
formance was still obtained. On the basis of these drilling 
results and those for Westerly Granite, Minnesota taconite, 
and Tennessee marble with aluminum chloride, it is concluded 
that drilling with ZSC concentrations of inorganic salt solu- 
tions should result in improved penetration and bit life 
responses. Determination of the applicability of ZSC- 
controlled drilling to all types of surface charge modifiers re- 
quired testing of previously tested rock types with other types 
of surface charge modifiers such as organic cationic surfac- 
tants, cationic polymers, and nonionic polymers with some ca- 
tionic character. 

Cationic Surfactants 

Four cationic surfactants were tested as surface charge 
modifiers. Three of these surfactants were familiar quaternary 
ammonium salts: the 12-carbon dodecyltrimethyl ammonium 
bromide (DTAB), the 14-carbon tetradecyltrimethyl am- 
monium bromide (TTAB), and the 16-carbon hex- 
adecyltrimethyl ammonium bromide (HTAB). The fourth sur- 
factant was a commercially available low-molecular-weight 
cationic polymer, Percol 402. DTAB, TTAB, and HTAB drill- 
ing solutions were prepared in DDIW and tested on Sioux 
Quartzite; therefore, the drilling performances obtained with 
these additive solutions were compared with the baseline drill- 
ing performance for Sioux Quartzite with DDIW. Percol 402 
drilling solutions were prepared in Erie Mining Co. mine well 
water and were tested on Minnesota taconite. Percol 402 drill- 
ing performance was therefore compared with the baseline 
drilling performance for Minnesota taconite with Erie Mining 
mine well water. Drilling tests were conducted with surfactant 
solution concentrations below, at, and above the ZSC concen- 
tration. 

Penetration and bit life effects for drilling Sioux Quartzite 
with DTAB, TTAB, and HTAB in DDWI are given in table 1 
and plotted versus surfactant concentration in figures 8 and 9. 
Drilling with DTAB at a concentration of 9.8 x 10" 4 mol/L, 
which is very near the ZSC concentration of 9.4 x 10" 4 mol/L, 
produced a maximum increase in penetration of 1 18 pet and a 
near maximum in bit life of 100 pet (figs. 8A-9A). Penetration 
effect improvements for TTAB and HTAB at their respective 
ZSC concentrations of7.2xl0 -5 mol/L and 1.6x 10' 6 mol/L 
were both 88 pet, while bit life extensions were 87 and 56 pet, 
respectively (figs. 8B-9B, 8C-9C). 

Penetration and bit life effects for Percol 402 are given in 
table 1 and plotted as a function of concentration in figures 5C 
and 6C, respectively. Because the exact equivalent molecular 
weight of Percol 402 was not known, solutions were made up 
in parts per million expressed as volume per volume. Drilling 



102 



with the very dilute solution of Percol 402, which produces the 
ZSC condition at 0.64 ppm, resulted in the best drilling per- 
formance: a 175-pct improvement in penetration effect and a 
141 -pet improvement in bit life. 

It is concluded from the drilling results for cationic sur- 
factants that drilling with ZSC-concentration solutions of 
these surfactants also gives rise to simultaneous maximum 
penetration effect and maximum bit life. 

Acid-Base Solutions 

Surface charge neutralization also occurs at the isoelectric 
point (1EP) that is achieved by adjusting the pH of the water 
with either acid or base, depending upon whether the IEP pH 
is lower or higher than the incipient pH of the water. For 
magnetite, the IEP pH is about 5.5. Therefore, drilling of 
Minnesota taconite was conducted with solutions whose pH's 
were above, at, and below the IEP pH value of 5.5. Penetra- 
tion and bit life effects are given in table 1 and plotted as a 
function of pH in figures 5B and 6B. Buffered solutions were 
employed to ensure that the desired pH was maintained 
throughout the drilling test. This was important since the ma- 
jor chemical byproduct of rock drilling has been found to be 
hydroxide ions (OH). Monitoring the pH of both the influent 
and effluent streams when drilling with DDIW alone has 
shown that drilling raises the drilling fluid pH as much as 3 to 4 
pH units. Drilling with a pH 5.5 buffered solution (acetic acid- 
sodium acetate) resulted in simultaneous increases of 279 pet 
in penetration and 218 pet in bit life compared with drilling 
using DDIW alone. One possible explanation for these much 
higher drilling performance improvements, which are about 
twice those for drilling Minnesota taconite with ZSC- 
concentration solutions of aluminum chloride, is that the ZSC 
condition is maintained throughout the test by the buffered 
solution. With the ZSC-concentration solution of aluminum 
chloride, this is probably not the case, as the hydroxide ions 
produced in drilling alter or vary the surface charge condition 
during the course of the drilling test. The results of these tests 
further point to the universal application of ZSC-controlled 
drilling irrespective of the type of additive employed to 
neutralize the surface charge. 



Cationic Polymer 

Because drilling particulates flocculate in a ZSC- 
concentration solution, flocculation was thought to be partial- 
ly responsible for the enhanced drilling performance at the 
ZSC concentration. Therefore, two polymers (one cationic 
and one nonionic) that flocculate particulates primarily by a 
molecular bridging mechanism, and secondarily by a charge 
neutralization mechanism, were tested as drilling-fluid ad- 
ditives. 

The cationic polymer used as a drilling-fluid additive was 
the water-soluble, high-molecular-weight polyacrylamide 
(PAA). Solutions of PAA were prepared in Minneapolis tap 
water for test drilling in Sioux Quartzite. The drilling perform- 
ance of PAA was therefore compared with the baseline drilling 
performance of Sioux Quartzite in Minneapolis tap water. 
Solution concentrations of PAA below, at, and above the ZSC 
concentration were tested. Penetration and bit life effects for 
PAA are given in table 1 and are plotted as a function of con- 
centration in figures lOfi and llfi. As with other cationic ad- 
ditives, drilling with the ZSC concentration of PAA (0.25 
ppm) produced simultaneous maximum increases in penetra- 
tion and bit life effects, 334 pet and 187 pet, respectively. Also, 



it should be noted that the PAA drilling results clearly indicate 
that flocculation is not responsible for enhanced ZSC- 
controlled drilling. While flocculation with PAA occurs at all 
concentration levels above 0.1 ppm, the enhanced drilling per- 
formance is achieved only at the ZSC concentration of 0.25 
ppm. The results of these tests with the cationic polymer also 
indicate the universality of ZSC-controlled drilling irrespective 
of the type of additive employed to neutralize the surface 
charge. 

Nonionic Polymer 

The nonionic polymer used as a drilling-fluid additive was 
the water-soluble, high-molecular-weight polymer poly- 
ethylene oxide (PEO). Drilling solutions of PEO were 
prepared in Minneapolis tap water for testing Sioux Quartzite 
and in Erie Mining Co. mine pond water for testing Minnesota 
taconite. The drilling performance of PEO was compared with 
the respective baseline drilling performance for Sioux Quart- 
zite in Minneapolis tap water and Minnesota taconite in Erie 
Mining mine pond water. Although PEO is available in a 
range of molecular weights from 100,000 to 6 million, drilling 
tests were conducted only with the 5-million-molecular-weight 
nonionic polymer because ZSC concentrations (parts per 
million) were found to be the same as for these other PEO 
molecular-weight varieties. Penetration and bit life effects for 
PEO in drilling Sioux Quartzite and Minnesota taconite are 
given in table 1 and are plotted as a function of concentration 
in figures 10C and 1 1C for Sioux Quartzite and SD and 6D for 
Minnesota taconite. 

PEO is an unusual surface charge modifier. Because it is 
nominally nonionic, it was expected not to neutralize the rock 
surface charge arid, therefore, not to produce enhanced ZSC- 
controlled drilling. This expectation was due to the observa- 
tion that zeta potential determinations of Sioux Quartzite par- 
ticles in commercial nonionic surfactants, such as Tergitol 
NPX, Surfynol 465, and hexaethyl cellulose (a high-molecular- 
weight polymer), showed no effect on the surface charge of the 
Sioux Quartzite particles at any concentration regardless of 
molecular weight. Zeta potential determinations of Sioux 
Quartzite particles in solutions of ever-increasing concentra- 
tion of PEO, however, showed that PEO, like the cationic ad- 
ditives (inorganic salts, cationic surfactants, and cationic 
polymers), alters the zeta potential from an initial negative 
value to zero. But, unlike the cationic additives, PEO does not 
produce a positive zeta potential at higher concentrations. In- 
stead, at higher concentrations, the ZSC condition is main- 
tained. PEO appears to have some cationic behavior in water, 
which allows it to neutralize the negative surface charge on 
rocks and produce the ZSC condition. In the absence of a 
charged rock surface, the cationic behavior, however, is not 
active and the rock surface charge remains neutral and is not 
made positive. 

Comparison of performance graphs of the penetration 
and bit life effects as a function of PEO concentration for 
Sioux Quartzite or Minnesota taconite (figs. 10C-1 1C, 5D-6D) 
with similar plots using cationic surfactants (figs. 8-9, 5C-6C) 
and inorganic salts (figs. 2-3, 5A-6A) shows why PEO is the 
best drilling performance enhancer. For Sioux Quartzite, 
penetration effects of over 350 pet and bit life effects of over 
235 pet were attained with PEO (from 3 to 125 ppm), and for 
Minnesota taconite, penetration effects of over 650 pet and bit 
life effects of over 420 pet were attained with PEO (from 12.5 
to 125 ppm) compared with the much lesser penetration and 
bit life effects obtained with ZSC-concentration solutions of 
the cationic surfactants or inorganic salts. In addition, there is 



103 



a wide range of concentrations of PEO that produce a max- 
imum penetration effect, compared with the single concentra- 
tion of cationic additives that produces the maximum penetra- 
tion effect. 

It is concluded on the basis of the tests with PEO that this 
polymer is the best additive for accomplishing ZSC-controlled 
drilling. Not only does PEO produce the enhanced ZSC drill- 
ing phenomenon over a wide range of concentrations, it also 
produces much greater improvements in bit life and penetra- 
tion. It is further concluded on the basis of the tests with PEO 
and the cationic additives that establishment of the ZSC condi- 
tion is the most important factor in enhanced drilling. 



EFFECT OF DRILLING-FLUID WATER ON 
ZSC-CONTROLLED DRILLING 

DDIW was used as the baseline comparison water in ini- 
tial drilling tests to clearly show any additive effect without ad- 
ditional spectator ions, such as those found in tap water or 
mine water, to mask the effect. Determining the applicability 
of ZSC-controlled drilling with regard to water type required 
testing of previously tested rock types with other water types, 
such as tap water and mine water. While the pH of the DDIW 
used in ZSC-controlled drilling research was constantly in the 
range 5.3 to 6.0, the pH of Minneapolis tap water ranged from 
7.3 to 7.7 and the pH of Erie Mining Co. mine pond and well 



waters ranged from 7.0 to 7.5 and 7.9 to 8.0, respectively. 
Therefore, in using aluminum chloride as the additive in tap 
water or in either of the mine waters, a pH adjustment of these 
waters was required before aluminum chloride was added, to 
prevent precipitation and flocculation of the Al + 3 ions in solu- 
tion as aluminum hydroxide [Al(OH) 3 ]. Acidifying the tap 
water to pH 3.8 with hydrochloric acid resulted in only a 
negligible decline in drilling performance on Sioux Quartzite, 
compared with plain tap water without the addition of acid. 

Penetration and bit life effects for drilling Sioux Quartzite 
with aluminum chloride solutions made from acidified tap 
water compared with drilling with acidified tap water only are 
given in table 1 and graphically displayed in figures \0A and 
11/4. Drilling with the ZSC concentration of 1.8 x 10 -4 
mol/L aluminum chloride in pH-adjusted tap water resulted in 
a maximum increase of 1 14 pet in penetration and a near max- 
imum of 60 pet in bit life compared with pH-adjusted tap 
water alone, thereby showing that enhanced drilling perform- 
ance could also be obtained with aluminum chloride in 
acidified tap water. 

It is concluded on the basis of these data, and results for 
drilling Minnesota taconite with Percol 402 and PEO solutions 
(figs. 5C-6C, 5D-6D) in mine well water and mine pond water, 
respectively, and Sioux Quartzite with PAA and PEO solu- 
tions in tap water (figs. 105-1 IB, 10C-11C), that ZSC- 
controlled drilling is applicable to a wide range of drilling-fluid 
water characteristics. 



FIELD DRILLING TEST RESULTS 



MINNESOTA TACONITE 



OTHER SILICATE ROCK SUITES 



Field drilling tests are being conducted on taconite at 
various mining operations in the Mesabi Iron Range of north- 
ern Minnesota. One of these tests consisted of drilling 
numerous 50- ft blastholes, at 30- by 30-ft spacings, with 15-in- 
diameter rotary tricone bits on two adjacent benches of cherty 
taconite. Drilling performance using an air-water flushing mist 
was compared with performance using an air-PEO solution 
mist for drilling particulate removal and dust control. The first 
bench was drilled solely with water, in 1984, consuming six 
tricone bits, and has subsequently been blasted away. The sec- 
ond bench was drilled with water (first 40 holes) and PEO 
(next 100 holes) in 1987. Figure 12 shows the penetration rate 
results for this test. The average penetration rate for the 165 
holes drilled with water in 1984 was 0.55 ft/min, while the 
average penetration rate for the 40 holes drilled with water in 
1987 was 0.56 ft/min. The average penetration rate for the 100 
holes drilled with PEO in 1987 was 0.93 ft/min. Assuming that 
the penetration rate for the section of bench 2 drilled with 
PEO in 1987 would be about 0.55 ft/min if the bench had been 
drilled with water, then drilling with PEO results in a 69-pct in- 
crease in penetration rate. Also, for water drilling, the average 
life for two bits was 2,745 ± 12 ft, while for PEO drilling the 
average life for two bits was 3,468 ±40 ft: a 26-pct increase in 
bit life. An added benefit in using the air-PEO solution mist in 
rotary tricone drilling of taconite is that it drastically reduces 
the amount of dust generated. 

It is concluded from these initial rotary tricone drilling 
tests that drilling with a mist of ZSC-concentration solutions 
of PEO in air results in increased penetration rates and in- 
creased bit life. Field drilling tests are continuing to further 
validate these initial results. 



Field testing of PEO was also done under cooperative 
research programs with HDRK Mining Research Limited of 
Copper Cliff, Ontario, and Centre in Mining and Mineral Ex- 
ploration Research (CIMMER) at Laurentian University in 
Sudbury, Ontario, under an umbrella agreement between the 
Canada Centre for Mineral and Energy Technology 
(CANMET) and the U.S. Bureau of Mines. 

In the HDRK cooperative research program, 3 weeks of 
diamond core drilling and percussive drilling tests in quartzite 



6 0.8 



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a Waler drilling, 1987 


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60 80 100 120 

NUMBER OF HOLES DRILLED 



Figure 12.— ZSC control of penetration rate in field drilling 
of Minnesota taconite with PEO. 



104 



and quartz-mica schist were conducted at both underground 
and surface rock exposures under near-production conditions 
at the INCO Thompson Mine in Thompson, Manitoba. Dia- 
mond core drilling tests both on the surface and underground, 
under certain conditions, demonstrated improved bit life with 
ZSC-concentration PEO solutions. But, because of the large 
data scatter and small sampling population, the drilling results 
did not conclusively demonstrate the enhanced drilling per- 
formance with ZSC-concentration PEO solutions. The wide 
scatter in diamond core drilling bit life (footage drilled) can be 
attributed to inconsistencies in rock morphologies for holes 
drilled on the surface and variability in bit characteristics for 
holes drilled underground. 

Drilling performance with ZSC-concentration PEO solu- 
tions was also determined in long-hole percussive drilling on 
the surface with 51-mm-diameter cross bits and underground 
with 51-mm-diameter button bits. Figure 13 shows an unused 
cross bit, a cross bit after drilling 100 ft in quartzite with 15 
ppm PEO solution, and a cross bit after drilling 100 ft in the 
same rock with water alone. It can be seen that the wear on the 
PEO drilled bit is less than on the water drilled bit. To substan- 
tiate this visual observation, an optical comparator was used to 
sketch the 2-D wear profile trace for each of the bits. A typical 
comparator trace is shown in figure 14 for one of the carbide 
inserts from each of the three bits shown in figure 13. The area 
of these traces was measured and then used to calculate the dif- 
ference in the wear of the carbide inserts. For the same 
distance of rock drilled, the cross bit used with polymer solu- 
tion was worn 42.6 pet, whereas without polymer the wear was 
greater at 48.7 pet. 

Typical comparator traces for one of the carbide button 
inserts from an unused button bit, a button bit used to drill 200 
ft of quartz-biotite schist with 15 ppm PEO solution, and a 
button bit used to drill 200 ft of quartz-biotite schist with 
water alone are shown in figure 15. The area of these traces 
was measured and then used to calculate the difference in the 
wear of the carbide button inserts. For the same distance of 



rock drilled, the button bit used with polymer solution was 
worn only 5.3 pet, whereas without polymer the wear was 12.1 
pet, more than twice as much, showing a significant improve- 
ment using polymer solution. 

In terms of penetration rate, tests using both the cross bit 
and button bit percussive drills showed no appreciable dif- 
ference in average penetration rates when the use of polymer 
solution was compared with use of water alone as the drilling 
fluid. Cross bits advanced at about 1.9 to 2.2 ft/min, button 
bits at 1.8 ft/min in the respective rock faces. 

Therefore, it is concluded from these data that drilling 
with ZSC concentrations of PEO solutions extends the life of 
both cross and button percussive bits, in spite of the lack of an 
improvement in penetration advance. Should these percussive 
bits be drilled to exhaustion, about three to five times longer 
than tested, it is expected that the polymer solution would fur- 
ther retard the carbide wear, permitting longer usage time per 
bit and longer intervals between carbide regrinds. 

In the CIMMER cooperative research program, Longyear 
Canada, under the guidance of staff from Laurentian Univer- 
sity and the Bureau, tested PEO as a drilling-fluid additive in 
exploration core drilling in granite from 3,000 to 5,000 ft 
below the surface, using BQ-size core bits. There was no 
measurable change in instantaneous penetration rate; 
however, bit life (total footage drilled) was improved 2.3- to 
3.1 -fold when drilling with PEO solutions, compared with 
drilling using water alone. Also, drill string vibration was 
reduced when PEO was used, compared with Dromus B solu- 
ble oil for this lubrication purpose. In addition, below 4,000 
ft, the diamond-impregnated bits remained continually sharp; 
they did not have to be periodically sharpened by lowering the 
drill rotational speed momentarily (increasing torque) to 
abrade away the dull surface diamonds and expose a new layer 
of sharp diamonds. It is concluded from these data that bit life 
is extended and drill string vibration reduced in diamond core 
drilling using ZSC concentrations of PEO solution. 




Figure 13.— Three cross bits. From left to right: unused bit, bit after drilling 100 ft quartzite with PEO, and bit after drilling 100 ft 
quartzite with water. 



105 



KEY 
Area, pet Wear, pet 

A 100.0 

B 57.4 42.6 

C 51.3 48.7 






KEY 




Area, pet 




Wear, pet 


A 100.0 







B 84.6 




15.4 


C 81.7 




18.3 

i-A 
/ /~B 

i / r c 


Reference 


line 


SIDE VIEW 



Figure 14.— Cross-sectional view showing optical comparator wear profile of typical 18-mm tungsten carbide wedge inserts 
from 51-mm cross bit. 



Toward 
center 
of bit 




Edge 
of bit 



KEY 

Area, pet Wear, pet 

A 1 00.0 0. 

B 94.7 5.3 

C 87.9 I2.I 

Figure 15.— Cross-sectional view showing optical comparator wear profile of typical 12mm tungsten carbide buttons from 
51-mm button bit. 



106 



POTENTIAL APPLICATIONS FOR IN SITU MINING TECHNOLOGY 



Development of an in situ mining production field re- 
quires diamond core drilling to characterize an ore body and 
rotary drilling to create boreholes for injection and produc- 
tion. Thus, rock drilling is absolutely essential to in situ mining 
methodology. 

It has been shown that PEO as a drilling fluid additive has 
improved drilling performance in a variety of diamond coring 
and rotary drilling applications. Since all of the rocks tested 
thus far (silicates, carbonates, oxides, and sulfide ores) can be 
treated by dilute PEO solution to achieve a ZSC condition, it 
would be a practical undertaking to incorporate this polymer 
into an in situ mining development plan as a drilling-fluid ad- 
ditive for all drills involved. 

An example of an applicable in situ mining development 
is in the Santa Cruz ore deposit in Casa Grande, AZ, where the 
Bureau is conducting an in situ mining field demonstration 
project jointly with ASARCO Santa Cruz Inc. and Freeport 
Copper Co. The ore deposit being drilled is composed of a 
variety of copper oxide minerals hosted in the Oracle Granite 
and an adjacent quartz monzonite porphyry. This deposit is 
probably typical, as far as drilling for copper mineralization is 
concerned. The overall grade for the deposit is about 0.7 pet 



copper; the deposit has never been worked by underground or 
surface mining, so it will be developed by surface drilling. 
Plans call for wire-line diamond drilling to obtain 2'/2-in core 
down to 2,000 ft; later these 4-in-diameter drill holes will be 
used as the injection wells. Recovery wells will be drilled by 
rotary drilling to the same depth, starting out as 12-in holes 
and tapering to 6-in holes at depth. PEO polymer solutions 
can be used in both instances, as the drills being used are nor- 
mally equipped to handle fluids, and ZSC concentrations can 
be determined for each of the rock types encountered. 6 
Polymer concentrate can be added to the drilling water mixing 
system at the driller's discretion. 

Other deposits that the Bureau may attempt to initiate for 
in situ mining research include cherty iron formations for 
manganese oxides and carbonates in the Cuyuna Range of 
Minnesota, the Duluth Gabbro formation for copper, nickel, 
and platinum-group-element sulfides, and a variety of 
volcanics, elastics, sediments, argillites, carbonates, and fer- 
rous and siliceous rocks for gold and silver deposits. 
Laboratory results with PEO suggest that the polymer can be 
used confidently in drilling and should perform well with all 
rocks encountered. 



SUMMARY AND CONCLUSIONS 



Laboratory and field ZSC-controlled drilling tests have 
been conducted to determine the boundary conditions for the 
enhanced drilling phenomenon. Laboratory drilling tests were 
performed on Sioux Quartzite, Westerly Granite, Minnesota 
taconite, and Tennessee marble, using diamond-impregnated 
coring bits or tungsten carbide spade bits. Drilling fluids were 
prepared from chemical additives such as inorganic salts, ca- 
tionic surfactants, acids, cationic polymers, or nonionic 
polymer in either DDIW, mine water, or tap water at concen- 
trations below, at, and above the ZSC concentration. 

Enhanced ZSC-controlled drilling performance was 
observed in laboratory drilling of high-silicate rocks, Sioux 
Quartzite and Westerly Granite; a medium-silicate rock, Min- 
nesota taconite; and a nonsilicate rock, Tennessee marble; and 
in the field drilling of granite, taconite, and quartz-biotite 
schist, thereby establishing the applicability of ZSC-controlled 
drilling to a broad range of rocks. 

The applicability of ZSC-controlled drilling using drag bit 
types was shown by the diamond-impregnated core drilling of 
Sioux Quartzite, Westerly Granite (5-6), and Minnesota 
taconite, and the tungsten carbide spade bit drilling of Ten- 
nessee marble. Field testing with percussive and rotary tricone 
and diamond-impregnated coring bits showed the applicability 
of ZSC-controlled drilling to these bit types. 

Enhanced drilling performance has been obtained in the 
laboratory with solution concentrations of inorganic salts, ca- 
tionic surfactants, cationic polymers, and nonionic polymer 
with some cationic character and in the field with PEO with 
some cationic character. All of these additives neutralized the 
rock surface charge, thereby establishing that the ZSC condi- 
tion is the most important factor in ZSC-controlled drilling. 

The applicability of ZSC-controlled drilling with respect 
to drilling-fluid water source or composition was established 
by achieving enhanced drilling performance with DDIW, mine 
water, and both plain and acidified tap water in the laboratory 



and with mine waters in the field. Because of the increased 
amounts of anions in the mine and tap waters, more additive 
was required for surface charge neutralization in laboratory 
tests when using these waters in place of DDIW. 

Maximum penetration improvements obtained for ad- 
ditives compared with baseline water penetration results in the 
laboratory ranged from 84 pet for Tennessee marble drilled 
with a near-ZSC concentration of aluminum chloride in 
DDIW to over 650 pet for Minnesota taconite drilled with a 
wide range of ZSC concentrations of PEO in mine water. 
Maximum bit life improvements obtained for additives com- 
pared with baseline water bit life results in the laboratory 
ranged from 56 pet for Sioux Quartzite drilled with a ZSC con- 
centration of HTAB in DDIW to over 400 pet for Minnesota 
taconite drilled with a wide range of ZSC concentrations of 
PEO in mine water. 

The laboratory tests demonstrated that PEO with partial 
cationic character (polarizable structure) in aqueous solutions 
was the best additive for accomplishing ZSC-controlled drill- 
ing. Unlike the cationic additives, which achieved the ZSC 
condition at only one concentration, PEO maintained the ZSC 
condition over a wide range of concentrations because of its 
polarizable structure. Thus, with PEO, the criticality of the 
solution concentration during drilling is greatly diminished. In 
addition, use of PEO resulted in much greater improvements 
in bit life and penetration than use of the cationic additives. 

Initial field drilling results with ZSC-concentration solu- 
tions of PEO have also demonstrated enhanced drilling per- 
formance, compared with field drilling results with water 



6 The Bureau has developed a brief videotape and instruction sheet detailing 
the proper mixing procedure to get polymer into solution and in making dilu- 
tions. A copy of the video and instruction is available upon request from the Ad- 
vanced Mining Division, Twin Cities Research Center, Bureau of Mines, Min- 
neapolis, MN. 



107 



alone. In field diamond core drilling in granite, 2.3- to 3. 1-fold 
improvements in bit life were obtained. In field percussive 
drilling of 100 ft of quartzite with cross bits, bit wear was 
slightly reduced. In field percussive drilling of 200 ft of quartz- 
biotite schist with button bits, bit wear was reduced by more 
than half. In field rotary tricone drilling in taconite, penetra- 
tion rate was increased 69 pet (0.93 ft/min versus 0.55 ft/min) 
and bit life was increased 26 pet (3,400 ft versus 2,700 ft). 



These initial field drilling results are encouraging but need to 
be validated by further field drilling tests. 

However, development of the PEO polymer as a drilling 
fluid is far enough along that PEO can be used readily for ex- 
ploration, quarry, blasthole, and production drilling, as well 
as in the development of an in situ mining well field where 
drilling in hard rock is a necessary aspect of mining. 



REFERENCES 



1. Watson, P. J., and W. H. Engelmann. Chemically Enhanced 
Drilling. An Annotated Tabulation of Published Results. BuMines IC 
9039, 1985, 53 pp. 

2. Rehbinder, P. A., L. A. Schreiner, and K. F. Zhigach. Hardness 
Reducers in Drilling. A Physico-Chemical Method of Facilitating the 
Mechanical Destruction of Rocks During Drilling. Akad. Nauk SSSR, 
Moscow, 1944; transl. by Counc. Sci. and Ind. Res. (CSIR) (now 
CSIRO), Melbourne, Australia, 1948, 163 pp. 

3. Shepherd, R. Improving the Efficiency of Rotary Drilling of 
Shotholes. Trans. Inst. Min. Eng. (England), v. 133, pt. 11, Aug. 
1954, pp. 1029-1048. 

4. Westwood, A. R. C, N. H. Macmillan, and R. S. Kalyoncu. 
Chemomechanical Phenomena in Hard Rock Drilling. Trans. Metall. 
Soc. AIME, v. 256, 1974, pp. 106-111. 



5. Engelmann, W. H., P. J. Watson, P. A. Tuzinski, and J. E. 
Pahlman. Zeta Potential Control for Simultaneous Enhancement of 
Penetration Rates and Bit Life in Rock Drilling. BuMines RI 9103, 
1987, 18 pp. 

6. Pahlman, J. E., W. H. Englemann, P. A. Tuzinski, and P. J. 
Watson. ZSC-Controlled Drilling For Enhanced Penetration and Ex- 
tended Bit Life. BuMines RI (in press). 

7. Krech, W. W., F. A. Henderson, and K. E. Hjelmstad. A Stand- 
ard Rock Suite for Rapid Excavation Research. BuMines RI 7865, 
1974, 29 pp. 



C 2H 89 



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